Jap (as a derogatory term for Japanese):

The term Jap was a neutral demonym in the late 19th century but it got pejorated during the WWII.

In America, the term Jap came into wide use in 1860 to refer to the members of the Japanese embassy to the United States (see "Japs in 19th-century popular usage"). In newspapers, magazine articles, and nonfiction accounts of all kinds it was mostly just a breezy abbreviation, and in its earliest usages referred to specific groups--the embassy, or a particular troupe of Japanese acrobats touring the U.S. Only gradually did "the Japs" become a larger category. [College of Liberal Arts and Sciences]

Dating back to the late 19th century, use of this term was originally neutral. But because the Japanese were the enemy during World War II, the term Jap became derogatory. [Dictionary.com]

Jap was not considered primarily offensive; however, during and after the events of World War II, the term became derogatory. Nisei veterans who served in World War II were shunned with signs that read No Japs Allowed and No Japs Wanted, denied service in shops and restaurants, and had their homes and property vandalized. [Wikipedia]

— From Pinterest.com

— From Pinterest.com

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According to Japan Talk, the term Jap was not offensive before the World War II, to quote:

Q: Is the word Jap considered derogatory?

Yes, this phrase is widely considered a racial slur. Much of the English speaking world and the Japanese themselves consider this to be a derogatory term. If you use this word to describe Japanese people or culture — you are likely to offend.

The United States

In the United States, the word 'Jap' was sometimes used as a contraction of 'Japanese' before WWII. It wasn't considered derogatory. During the war, the word was widely used in an extremely derogatory way. This included government propaganda. As a result, the word was widely considered a racial slur after the war. Its use dropped dramatically. Today, it's considered extremely offensive by Japanese Americans.

Canada

Japanese Canadians consider the term offensive.

UK

The word Jap is generally considered derogatory in the UK. Britain is home to the largest Japanese community in Europe who consider the term offensive. The Japanese Embassy in London went after a London newspaper for using the term in 2011. The following statement came from the embassy at that time: > Most Japanese people find the word Jap offensive, irrespective of the circumstances in which it is used. ~ Embassy of Japan in the UK

Singapore

In Singapore, the word Jap is widely used as a contraction of Japanese and isn't considered derogatory.

Japan

The Japanese are very aware that the word Jap is often used in a derogatory sense. Using this word is likely to be hurtful.

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From densho.org:

In the 1950s, Shosuke Sasaki launched a campaign to have the word “Jap” re-classed as a racial slur and eliminated from print media. He would continue that work for the better part of the next 20 years.

As early as 1880, the English Oxford dictionary noted “Jap” as a colloquial term of abbreviation. Though it may have started out as a neutral term, it took on increasingly negative connotations as anti-Japanese sentiment grew. It was used as a slur against Japanese immigrants and, later, in reference to Japanese American citizens. Following World War II, the use of the word became even more opprobrious but was still in wide use.

In a letter to the Executive Committee of the Newspaper Guild of New York, Sasaki requested that Newspaper Guild Executive Committee place “Jap” in the same category as other racial epithets, discouraging use in newspapers and magazines. His arguments included:

• “The term 'Jap' is regarded by the Japanese as an epithet of derision. Its use is resented by all Japanese and persons of Japanese ancestry.”
• “The excuse that the term 'Jap' is usually used without any derogatory intention is pointless. It frequently has been and is being used with the connotation of contempt. Furthermore, not even a moron would persist in calling a person by any name which that person considered offensive, when at the same time the goodwill and friendship of that person were desired.“
• “The use of the term 'Jap' completely nullifies in the eyes of the Japanese all American claims to being the world’s champion of human dignity.”

In February 1952 , the New York Newspaper Guild passed a resolution and sent a letter sent to all New York publishers. But the resolution was not as effective as Sasaki had hoped and “newspapers continued to use the epithet as freely as ever.” Sasaki mounted a letter-writing campaign to the offending papers and organized protests with the help of the New York JACL.

[Densho.org]

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This American propaganda poster, created in the months following the bombing of Pearl Harbor by the Japanese, is a prominent example of name calling. In this poster, the Japanese are derogatorily depicted as rats while the words Jap Trap is prominently displayed in bold, with quotation marks, near the bottom of the poster. During the Second World War, many Americans used the term “Jap” as a derogatory term to refer to the Japanese, mainly because of the word’s ability to create perfect rhyme with other words and thus, create easy to memorize slogans such as Let’s blast the Jap clean off the map and Jap Trap. By referring to the Japanese as “Jap” in this poster, the propagandist aims to brew anti-Japanese sentiment among its American audience. The rat itself, with its wrinkled, hairless skin, buck teeth, and clawed hands is extremely displeasing to the eye and looks more like a vermin than a human. Coupled with the stereotypical manner the propagandist drew this poster, with its narrow eyes, black hair, circular glasses, and shriveled skin, and the slogan, Jap Trap, it is hard not to associate the Japanese with a negative image after viewing this poster. The propagandist uses this extremely insulting and disgusting portrayal of the Japanese to associate the “Japs” with a negative image of a vermin and brew hatred among the American people. [PropangandaProjectJustin]

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Some other anti-Japanese posters:

— From HistoryHit

— From Wikipedia

— From Pinterest.com

During the WWII, they were also referred to as Yellow Bucktoothed:

— From Japanese-Canadians - Weebly

— From Japanese-Canadians - Weebly

References:

• Wikipedia
• Spot a Jap - DailyMail UK
• Japan Talk
• Densho.org
• Huffpost
• History Hit
• Japanese-Canadians - Weebly

'Swede' is not a shortening of the correct description. 'Swede' is the proper description. (The Swedes live in Sweden.)

'Finn' is not a shortening of a description. 'Finn' is the proper description. (The Finns live in Finland.)

[For the correct spelling of 'Finns' I have consulted en.biginfinland.com]

'Scots' is not a shortening of a description. 'Scots' is the proper description. (The Scots live in Scotland.)

But the Japanese live in Japan.

And 'Jap' is a wrongful shortening of their proper description.

As such, the shortening is a familiarity which is misplaced in a formal setting: for example when someone of a foreign country wishes to name them.

Dictionary discussions of 'Jap'

According to the resources I checked, two derogatory slang terms used specifically in reference to Japanese (and Japanese American) people were popular during and after World War II, both involving shortenings of longer words: "Jap" (from Japanese) and "Nip" (from Nippon). There seems to be some disagreement about which term was then considered more offensive, but both are beyond the pale today—at least in the United States.

Here are the entries for both terms in Paul Dickson, War Slang: American Fighting Words and Phrases from the Civil War to the Gulf War 1994):

Jap. (1) A Japanese. (2) Anything Japanese.

...

Nip. Japan; short for "Nippon." Racist by today's standards, this term was used routinely in broadcasts, news dispatches, and conversation during the war, as was the term "Jap."

And here are parts of much more detailed coverage of "Jap" and "Nip" in Philip Herbst, The Color of Words: An Encyclopædic Dictionary of Ethnic Bias in the United States (1997):

Jap. Pejorative dating from perhaps as early as the mid-nineteenth century for a Japanese person; also used for things made in Japan. In its early decades of use, the derogatory overtones may not have been as definite, and it is still less pejorative in Britain that in the United States. According to Merriam-Webster's Dictionary of English Usage (1989), derogatory use can be traced to West Coast newspapers during the first decade of the twentieth century.

...

During the [second world] war, Americans used the terms Nazi and Fascist to distinguish the German and Italian enemy from German Americans and Italian Americans, but no such terminology applied to the Japanese, all of whom—both American citizens and the foreign enemy—were "Japs" [reference omitted]. Similarly, while Americans fought Hitler, the "Japs," not Tojo or Hirohito, were their enemy during the war. The expression "After all, a Jap is a Jap" was used to justify the government's rounding up of Japanese Americans and sending them to relocation camps.

The editor of the New Republic once defended the publication's use of a headline that read "How to Gyp the Japs," arguing that the term is only a national nickname, similar to Yanks, Brits, or, at worst, Frogs. The Asian American Handbook, which cites this example, counters that words such as Jap and nippers are "not nicknames. They are slurs never to be used" (1991, 4.2).

In 1986, Sense of Congress Resolution 290 recommended to the Committee on Education and Labor that the standard abbreviation Jap (for Japan and Japanese) be replaced by the nonoffensive Jpn. In the 1990s, however, the racist slur stayed in force in response to Japanese success in American economic markets, as shown, for example, in a bumper sticker reading "No Jap crap for this American."

...

Nip, nip, nipper. Demeaning slang for a Japanese or Japanese American, from the Japanese word Nippon (Japan). It was used largely during World War II. "So you knocked down a Nip?" (question put to a pilot, in Nicholas Ray's Flying Leathernecks, 1951). It is still used today, sometimes for any East Asian.

As Herbst notes, Merriam-Webster's Dictionary of English Usage (1989) has a fairly extensive discussion of "Jap" as well:

Jap, Japanese. The word Jap is a clipped form of Japanese that has been around since the end o the 19th century. ...

Clipped terms are often felt to be somewhat less than respectful—even such inoffensive terms as prof and doc have been criticized—and clipped ethnic terms are doubly suspect. In the United States this term has a considerable history of pejorative use: according to evidence in our files it was used with derogatory intent by newspapers on the West Coast from the first decade of this century. ...

Americans are not unaware that the term was offensive. The first commentator to note this in print seems to have been [G.M.] Hyde [Handbook for Newspaper Workers] 1926, who did not specifically term it offensive but did not approve its use in newspapers. H. L. Mencken in The American Language in The American Language (the 1963 abridgement) says that Japanese-Americans on the West Coast had long objected to its use, but to little avail. And then World War II gave a great boost to the usage. No only did Jap have brevity to recommend it but it probably served a psychological purpose: [quotation omitted].

World War II is more than forty years behind us, and attitudes have changed. Spiro T. Agnew noted hanging attitudes in 1968:

People are getting too edgy, the Republican Vice-Presidential candidate believes, when they take umbrage at being called "Polacks" or "Japs: in a spirit of fun —Homer Bigart, N.Y. Times, 25 Sept. 1968

That sot of fun has seldom been shared in by the ethnic groups so designated.

Jap is generally either used disparagingly or taken to be offensive, even in innocent use. Unless your intent is to recreate the atmosphere of World War II for some reason, use Japanese.

I might add that the last mainstream U.S. political/legal figure I am aware of as having used the term Jap was John Wilson, John Ehrlichman's attorney at the Watergate Hearings in 1973, who used it in reference to Senator Daniel Inouye of Hawaii, as reported in the U.S. House of Representatives History, Art & Archives page dedicated to Inouye:

Within months of joining the investigation, Inouye’s name appeared in headlines across the country when John J. Wilson, the attorney representing former Nixon chief of staff, H. R. Haldeman, used a racial slur to describe the Senator from Hawaii. When asked by a United Press International reporter about a line of questioning from Senator Lowell Weicker of Connecticut, Wilson responded, "Oh, I don’t mind Senator Weicker.… What I mind is that little Jap," and gestured toward Inouye.

Standards regarding the acceptability of "Jap" and "Nip" may vary across the English-speaking world, but it is widely considered unacceptable in public discourse in the United States and evidently has a long history of denigrative use in this country.

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Derogatory use of 'Japs' in early U.S. and Australian newspaper articles

Whether very early instances of the use of "Jap" reflected the writer's intention for readers to understand it as a derogatory term is difficult to say—but when the term appears in conjunction with a pejorative modifier, it seems reasonable to conclude that the combined term was meant to be taken in a hostile sense. One of the earliest such combinations that I found in old newspaper articles was the phrase "heathen Jap[s]," which begins appearing in U.S. newspapers in the early 1870s. For example, from an untitled item in the Terre Haute [Indiana] Gazette (January 6, 1871):

The "Japs" are registered at the National, and we would advise any one interested in the art of short-hand writing to go there and inspect the register, and inform us what system of short-hand the "heathen Japs" use.

From an untitled item in the Donaldsonville [Louisiana] Chief (March 9, 1872):

We learn from an exchange the astounding intelligence that "foreigners are to be prohibited from buying Japanese itziboos." We don't know what those things are, but its abuse of foreigners to prohibit them from purchasing the articles if they feel inclined to do so. Have foreigners no rights which the heathen Japs are bound to respect—not even the right to spend their money as they see fit?

From "Exposition Topics" in the Gallipolis [Ohio] Journal (July 13, 1876):

So as Bill Nye went for that heathen Chinee who knew nothing of cards, the American dealers in Japanese goods are inclined to go for the heathen Japs, and are enthusiastic to support the Mongolian plank in the St. Louis platform. The Japs who sell their fancy goods are almost all of them dressed in the American fashion and speak English so as to be understood without difficulty, although it is impossible for any of them to throw off their strong accent, and impress every one as being fr more intelligent and progressive than their almond eyed neighbors.

From "Japanese Fun: The Heathen Idea About a Practical Joke," in the Indianapolis [Indiana] News (January 14, 1879):

The heathen Jap’s idea of a practical joke is of a ridiculously childish sort, judged by the Anglo-Saxon standard. A reporter of the Tokio Times gives some illustrations of the fun that can be had any evening in the streets by a waggishly inclined individual. ... The mad wag is to drop coins on the clothes of sleeping children, buy dolls for little girls who have broken theirs and are crying over the fragments, and then look for a dealer in cakes who has a sour countenance, and has evidently had a bad night's business. "Now," says the reporter, "you have an opportunity of upsetting a cake-dealer's calculations and knocking his notions of destiny endwise. There is nothing more amusing than upsetting a cake-dealers calculations and knocking his notions of destiny endwise. You call a couple of urchins who have been looking at his stock with hungry eyes and direct them to fall to. The dealer's face is a study. He is compelled to readjust all his convictions respecting fate and fortune in a moment." And this is the heathen Japs idea of a practical joke!

From an untitled item in the [New Market, Virginia] Our Church Paper (October 31, 1888):

The Japanese New Testament is now to be had in the pocket edition at the price of 2½ cents. The government has introduced it in all her schools. There are, at present, 31,000 public schools in Japan, with three million children. All these are reading now the gospel, and learn to know Christ. Those "heathen Japs" are far better off than the children in the public-schools in the "Christian country" of North America.

From "Japs 'go for' Rev. J. M. Rollins," in the Salem [Virginia] Times Register (April 3, 1891):

Japan is looked upon as the most progressive of all Eastern nations, but whilst they may readily adopt the manners and customs of the Caucasian race, they are loathe to accept Christianity. In the Baltimore Episcopal Methodist, of March 25th, appears a letter from Rev. James M. Rollins, who spent last summer in Salem, giving a thrilling account of a "narrow squeak" he had for his life in the Sano opera house, whilst endeavoring to preach to 1,500 heathen Japs, who instigated by Buddhists would have " put out his light," had it not been for the arrival of a plucky policeman. The cop drew his sword and rushed the minister to the hotel, and so he escaped. Now we suggest that the Missionary society send out a few reformed prize-fighters to convert the heathen by a little muscular Christianity.

Many instances of the "heathen Jap" trope from the nineteenth century seem relatively uncritical, but an instances from fairly early in the twentieth century is anything but benign—even though the thrust of the argument is to criticize Christians for not living up to their faith. From "Are We Christians," the [McConnellsburg, Pennsylvania] Fulton County News (November 19, 1912):

Certain pagan faiths teach that if you die in battle for your country, you will go straight to heaven; and during the late Russo-Japanese war, the Japs seemed to vie with each other in getting into the most dangerous position until they won the battle. During that war we were accustomed to comment upon the fanaticism of the heathen Japs; but the slaughter in those battles was as child's play when compared with what now takes place almost daily in "Christian" Europe. It was said that very few of the Japs and Russians knew what they were fighting about.

However, instances of "heathen Japanese" are even more common during the same period, and the contexts in which the two expressions appear are virtually interchangeable. Under the circumstances, it is difficult to say whether "heathen Jap" was meant any more pejoratively than "heathen Japanese."

Another set phrase with considerable currency in the U.S. during the late 1800s was "the wily Jap," applied sometimes to individual Japanese people and sometimes to the Japanese nation. Here are some examples of that usage from the 1890s.

From "Sauce for the Government," in the Seattle [Washington] Post-Intelligencer (September 20, 1890):

Customs Inspector Thompson looked over the baggage that came in with the passengers of the steamer Premier from Vancouver yesterday, and seized three large packages of Japanese sauce, which a wily Jap was trying to bring in as personal baggage. The stuff was put in the bonded warehouse and will be appraised and sold. Its value is about $2O.

From "Ingenuity of the Jap," in the Pittsburgh [Pennsylvania] Dispatch (February 22, 1891):

From "Another Deserter Caught," in the [Honolulu] Hawaii Star (October 24, 1894):

Tomo, the Japanese detective, made a good catch this morning. About a year ago he received notice from Kauai that a Jap laborer had escaped from Makaweli plantation, and instructions to arrest him when he reached Honolulu. Little Tomo soured the town for the wily Jap without success. A year passed and not until last night did the fellow turn up on the streets. Then Tomo was down upon him like a hawk. He was taken into custody and it has since developed that he had been working at Kahuku all the while.

A well-authenticated story which has just come from Japan would indicate that the wily Jap does not confine himself to the beaten paths in the exercise of his ingenuity. A gentleman broke the mainspring of his watch, which he took to the nearest jeweler. Somewhat to his surprise the watch was returned to him in as good running order as ever, and it ran all right until the rainy season set in, when it stopped. Being in the city of Tokio at that time, the traveler took the watch to an English workman, who on making examination was astonished to find that the cunning Jap had put in a spring made of bamboo, which, so long as it was kept dry, remained elastic, but during the wet weather had gathered dampness and lost its power.

From "The Wily Jap: Still Preparing to Take Possession of Hawaii," in the [Los Angeles, California] Herald (September 28, 1897):

Rear-Admiral Miller, in command of the Pacific station, who is now at Honolulu on the flagship Philadelphia, believes that Japan is planning to prevent the annexation of the islands to the United States, and has notified the Navy Department to that effect.

Yet another flurry of pejorative comments begin to appear in the late 1890s in connection with the expression "dirty Jap." From "Slopsey Nabbed Scotty: Why Battle-Axe Gleason's Valet Is Sore on a Cop" in the [New York] Sun (July 18, 1897):

"Slopsey [Parker, a policeman] had his club out," he [Edward Wilson, aka "Scotty"] said, "and was wild for me to fetch him a lick so's he could get back at me and kill me. He used language to me, all kinds. He was tantalizing me to make me poke him just once. It never worked. Then he put his hands down on his knees so, and squatted down and began hopping 'round me like a hoptoad wit' a pain on his inside.

"'Jap,' he yells, dancing 'round me, 'Jap! Ye're a Jap. A Jap. A dirty Jap.'

"I got mad finally, and, for the foist time, I told him, for fair, what I thought of him.

"Then he run me in, and I fixed things to run him off the force."

The nasty/vindictive deployment of "Jap" in this instance is unmistakable, both in the taunting use of "dirty Jap" and in the taunting use of unadorned "Jap." The oddness of the episode is only heightened by the fact that neither person involved was Asian.

From "Komatsu and the Coon: A Japanese Convicted of a Heinous Offense" in the Los Angeles [California] Herald (September 12, 1899):

She [Mrs. Johnson] testified that when she entered her room she found that the defendant [Komatsu] had thrown her daughter across a trunk and held his hand over her mouth, while her dress was disarranged. She picked up a pitcher and said: "I'll kill you, you dirty Jap. You have treated me dirty," and he replied: "She is 12 years old; 12-year-old girl all right, 13-year-old girl all right, 15-year-old girl all right." She didn't know what he meant. She then struck him with the pitcher and he ran away. The little girl corroborated her mother's story and said that the pitcher was broken in the chastisement inflicted upon the Jap. by Mrs. Johnson.

From a letter to the editor, titled "What Do the Policemen Say?" in the Honolulu [Hawaii] Republican (August 23, 1900):

Sir—I suppose if any white man should presume to parade the streets in a night dress open in front, allowing every zephyr from the pali to cause the exposure of his anatomy he would be subject to immediate arrest. A dirty Jap can, in a semi-nude condition, crowd the ladies off the sidewalk and grin at their discomfiture with perfect immunity. Such is life in Honolulu.

And from "Waiakea River for Government: The Mill Company Will Release It Rights to the Territory" in the [Honolulu, Hawaii] Hawaiian Gazette (October 9, 1900):

As part of the consideration the Government agrees to keep not less than two feet of water on the bar at all times. It will also clean up the whole river as far as the transferred area extends and see that all the shacks [inhabitated by Japanese squatters] near the mouth of the river are removed. They will also build a concrete pier from the bend of the river up to the old bridge. This can be used as a boat landing and public promenade, which will take the place of the present dirty Jap houses.

In this last instance, the correspondent may have intended the connection to be between "dirty" and "houses" rather than "dirty" and "Jap"—but for whatever reason, the person considered it appropriate to slip "Jap" into the longer phrase.

As Merriam-Webster's Dictionary of English Usage observes, hostile references to "Japs" are not hard to find by the early twentieth century. But again it is not clear that the term "Jap" by itself had an especially malevolent sense at that time. For example, a brief item in the Los Angeles [California] Herald of September 1, 1902 about a manager who disappeared with money for a payroll is titled "A Dishonest Jap," but to me the most objectionable aspect of the story isn't the word "Jap" but the reporter's emphasis on the manager's ethnicity in the first place. That focus would not have been any less obvious if the title had been "A Dishonest Japanese."

From "A Japanese Skips Out," in the [Honolulu, Hawaii] Evening Bulletin (December 17, 1897):

The Japs are rapidly falling into the ways of some other people. Unusual as it may appear a little brown man skipped out on the Australia in the regulation way of dead-beats and is now well on his way to San Francisco.

The man's real name is Taksuda, and he is a collector. Numbers of people know him; some wish they did not. Taksuda is a very, very wicked Jap. Last week he collected $225 from a poor man to pay another poor man for some bees. Taksuda pocketed the money himself. Another collection of $160 for an immigration company was not turned over. Altogether he misappropriated about $600.

From an untitled item in the Healdsburg [California] Tribune, Enterprise and Scimitar (April 26, 1900):

The immigration of Japanese boys into this country is not to be stopped if it lies in the power of newspaper to stop it. For years the little yellow men have been coming across the border, laborers traveling incognito as students, paupers as merchants, in an ever-increasing stream until it is said that 6026 have come through Victoria and Tacoma alone. Still larger numbers came through San Francisco, 100 being on one incoming ship. The Japs succeed in passing the immigration authorities by practicing time-honored tricks and deceits. Once in the land of the free the alleged students get work on the railroads or in the fruit sections, shutting out white labor. The Japanese are a greater menace to American labor than are the Chinese. They will work for less per day than will the Celestials. Then, again, they have a trick of imitation which enables them to copy American methods and mannerisms successfully. The uninitiated are easily deceived by the cunning Jap. Nearly all the Japanese are strong on politeness. It is related that a Japanese highwayman in Los Angeles said to his victim, "I'll rob you of ten dollars, if you please." By practicing their tricks of imitation and putting on their masks of politeness the Japs install themselves in the good graces of employers to the detriment of more honest, industrious but less docile white laborers. If the daily newspapers succeed in stopping Japanese immigration they will have done a good work for California.

From an untitled item in the Princeton [Minnesota] Union (December 13, 1906):

Again the country is saved. Several designing, treacherous Japs were frustrated in an attempt to secure admission to the fortifications at Sandy Hook, where they wanted to see the disappearing guns touched off!

From an untitled item in the [Prescott, Arizona] Weekly Arizona Journal-Miner (August 15, 1906):

Grieved American merchants are complaining to the state department with an air of childlike surprise that those wicked Japs in Manchuria are running affairs there purely for the benefit of Japan.

From "Monopoly the Scourge, Not Japanese Workers," in the Los Angeles {California] Herald (February 26, 1909):

The value of such a letter is the greater owing to the absurdly easy way in which the average citizen is cozened and fooled by the predatory "interests" and their reptile press. "Watch those 'treacherous' Japs," they cry, as they themselves rifle our pockets. And it works—fools that we are.

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Arguably nonpejorative use of 'Jap' in nineteenth-century newspapers

It would be misleading to suggest that all references to "Jap" in early newspaper accounts are negative. Here are some counterexamples, of positive and even admiring usage in U.S. newspapers. But even superficially positive framing of "Jap" can carry a disapproving or patronizing edge, as several of these examples indicate.

From "From the Centennial City: Philadelphia, March 18, 1876," in the Middlebury [Vermont] Register (March 28, 1876):

The Catholic Total Abstinence Association of Philadelphia are erecting near Machinery Hall an imposing and beautiful fountain of marble, at an estimated cost of $30,000 ; it will be a permanent ornament to the park. The little, industrious "Japs" have timber already on the ground, and are about erecting anothecr building near Memorial Hall ; and if the states of this Union are not on the alert, these little copper colored fellows from the land of "Tomeny" and the Mikado will carry off the building palm.

From "A Japanese Village: Madison Square Garden Transformed into a Suburb of the Town of Tilipu," in the Pontiac [Michigan] Gazette (December 18, 1885), reprinted from the New York Graphic:

Mr. G. T. Takamura, a very intelligent Jap and who speaks English fluently, though he has never been out of his country before, acts as interpreter and shows visitors about with the politeness that has come to be characteristic of his race. A Japanese indeed is by nature a gentleman, and as much unlike a Chinaman as it is possible to imagine. He also differs materially from the individual who wields a sword in "The Mikado."

From "The Japanese," in the Los Angeles [California] Herald (July 26, 1888):

The Japanese village at the Dime Museum is a good drawing card at that popular resort. It is very interesting to see the clever Japs at work on their unique and fragile wares. The artists at work in the several branches of art represented, are well worth seeing.

From "Japs Going ln for Watchmaking," in the Aspen [Colorado] Daily Times (March 15, 1894):

The enterprising Japs, ever on the lookout for the chance of turning a few honest pennies, have just gone in for watchmaking. A large company, with headquarters at Yokohama, has been started, and as labor in Japan is cheap, and the Japanese possess in a remarkable degree the mechanical skill which is so essential in the business, its prospects ought to be good.

From "The Migratory Jap," in the St. Paul [Minnesota] Globe (May 30, 1897):

A large portion of this [Japanese migratory movement] has turned to the occupation of Formosa, that rich spoil of the war with China, and Japanese have found in Hawaii opportunities for occupation that they have improved to an extent that is causing apprehensions in the breasts of the descendants of our missionaries, who have come to rule over the people of the Sandwich Islands. This fear comes not so much from the mere increase in numbers of the alert and industrious Japs, as from a fear that, with their keener intelligence, they will not long submit to the rule of the oligarchy that now styles itself a republic.

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Conclusions

It is surely inaccurate to assert that "Jap" has always been intended in a pejorative sense by every U.S. English speaker who has used it. At the same time instances where it has been used in a malicious way go back 150 years at least. The exoticism of Japan, its mystery as a country for many years closed to outsiders, the absurd but largely unchallenged caricatures of Japanese people in the hugely successful Gilbert & Sullivan operetta The Mikado (1884), and deep-seated racial bigotry in the U.S. during an extended period of unquestioned white political supremacy undoubtedly contributed to infusing "Jap" with, at best, a mixture of respect and disrespect.

Although it is not obvious to me when the U.S. press crossed the threshold into overall pejorative use of "Jap," there are numerous instances of such use before 1910. In particular "heathen Jap" appeared multiple times in the 1870s, "wily Jap" and "dirty Jap" multiple times in the 1890s, and such associations as "wicked," "cunning," "designing," and "treacherous" with "Jap" or "Japs" in the first decade of the 1900s. Under the circumstances, it's hard to see how "Jap" could be viewed in retrospect as a fundamentally neutral epithet in U.S. usage even 150 years ago.

A Google ngrams search shows that the term was rare until the late 1930s and its use fell off very rapidly in the late 1940s. It had a small revival in the late ’70s and early ’80s, when Americans began to fear that Japanese businesses were out-competing them.

A search of the examples in the Google Books corpus shows that most examples before the 1920s are simple abbreviations, usually followed by a period, such as, “Jap. Govt.”

In the 1920s, one sees the term appear as slang. For example, H.G. Wells’ The Land that Time Forgot, published in 1924, makes a not-very-convincing effort to write in a Californian voice:

Californians, as a rule, are familiar with ju-jitsu, and I especially had made a study of it for several years, both at school and at the gym of the Los Angeles Athletic Club, while recently I had, in my employ, a Jap who was a wonder at the art.

Since this is throwing a slang term into a sentence of British English to make it sound more superficially American, I’m hesitant to call it a good example of authentic usage.

There is one other novel from this period in Google’s corpus that uses the term, also of a servant.

A 1928 report by the US Department of Agriculture mentions

The Japanese Dwarf variety usually is referred to by the growers by the colloquial name “Jap Dwarf” and was formerly known also as Sterling Dwarf.

A 1925 register of Jersey cattle lists several prize cows with “Jap” in their names. Some unmistakably use it as a noun to refer to a person, such as “The Jap’s Owl.”

ETA: Thanks to @GEdgar for finding another example: the Montgomery Ward Catalog from 1898 onward frequently used “Jap” as an adjective to refer to silk, satin, pearls and so on, such as in this page from the 1920 catalog. Notably, the catalog was not trying to disparage the products it sold, but to exoticize them.

At this time, it would be a stretch to call the term respectful, but it wasn’t considered an insult until war propaganda began using it as one.

It was not offensive before World War II but after that most Americans and other people started to mock Japanese people and used "Jap" instead of the full form "Japanese". I have heard a lot about Japanese people. People say Japanese are very nice people but they are also too sensitive to criticisms. They could not stand being called by the slurs which included the term "Jap" that's why it is considered very offensive even in this period of time. Here is an article on Quora.com about their sensitivity: https://www.quora.com/Are-Japanese-people-usually-more-sensitive

I hope it helps.