Origin of "s--t-eating grin"

From what I understand, dogs would often follow horse-drawn wagons and eat the horse droppings. The taste caused the dogs to raise their heads and pull their mouths back in a sort of grin.

The phrase in slang dictionaries

The earliest slang dictionary coverage of the expression that I've been able to find is from Robert Chapman, New Dictionary of American Slang (1986), which offers this entry:

shit-eating grin n phr An expression of satisfaction; a gloating look: Go ahead, sit there with that shit-eating grin on your face—Earl Thompson/ stood there with a smug shiteating grin on his face—Fingered City

Nine years later, Robert Chapman & Barbara Kipfer, Dictionary of American Slang, third edition (1995) changes that basic definition only slightly:

shit-eating (or turd-eating) grin n phr by 1960s An expression of smug satisfaction; a stupid gloating look: [citations omitted]

Jonathon Green, Chambers Slang Dictionary (2008) provides a similar interpretation:

shit-eating grin n. (also cat-eating grin, crud-eating grin, shit-eating smile) {[from] shit-eating [1940s+, "1 a general term of disparagement 2 sly, duplicitous 3 toadying, subservient"} {1950s+} (orig. US) a smug, self-satisfied smile.

On the other hand, Tom Dalzell & Terry Victor, The New Partridge Dictionary of Slang and Unconventional English (2006) counters with this rather different entry:

shit-eating grin noun a broad smile, ingratiating and unctuous US, 1957 [Citations omitted.]

The Earl Thompson quotation in Chapman's 1986 dictionary seems likely to come from his novel A Garden of Sand (1971), while The Fingered City (1975) is a book by Denison Hatch. If these dates are correct, neither instance that Chapman cites is particularly close to the earliest reported occurrences in print of "shit-eating grin."

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Early instances of 'shit-eating grin' and allied phrases, 1957–1961

The earliest confirmed matches for the phrase that I've been able to find in various database searches are from the period 1957–1961. I think it's worthwhile to consider several less taboo versions of the expression from this same period, since these may be instances of censorship-enforced euphemism. Readers can judge for themselves whether these are in fact euphemisms or simply weakly related (or totally unrelated) similar-looking phrases.

From Len Giovannitti, The Prisoners of Combine D (1957) [combined snippets]:

Both men looked up from their potato-peeling job as Bob Rawlins came into the combine and sat down on the bench opposite. He had a a queer grin on his face and Storch and Zukerman knew he was set to spring the latest rumor. Rawlins was the hottest fighter pilot ever born; he was also from Texas. Everybody knew this two seconds after meeting him; and three seconds after meeting him, everyone knew he was the biggest liar ever born. He loved rumors, good or bad, and spent his days living on each new one he heard or could dream up.

"Okay, what's the shit-eating grin for? What's up now?" Storch asked, winking at Zukerman.

Rawlins kept the grin. "You mean you ain't heard?"

"No, we ain't heard, so tell us before you choke," Storch said.

The expression here could very well mean "self-satisfied grin," although it is not altogether clear what sort of "queer grin" the author actually imagines is playing on the character's face. It seems entirely possible that the aspect of the grin that the author is focusing on in this passage is its artificiality, unnaturalness, or forcedness.

From Gene Brown, The Locust Fire (1957) [combined snippets]:

"'Hold up there, Tom," says Pappy.

"The colonel stops like he's been shot in the back. He's a damn fine figure of a man, especially in the dress pinks, with them squared-off shoulders. 'Well?' he says.

" Old Pappy walks around in front of him. I guess the colonel got a good whiff of the juice, because he kind of sways back on his heels. But Pappy steps up close, with a big dirt-eating grin on his face, and starts running his hands up and down the lapels. Then he flicks a little imaginary dust off the colonel's epaulets and moves back a pace. "That's a real pretty suit, Tom,' he says. 'Ain't it a pity all these poor hungry boys here can't be wearing one just like it?'"

Whatever the "dirt-eating grin" looks like here, a grin of satisfaction or gloating doesn't fit the context particularly well. Pappy is drunk, so his grin may be good-humored in a sloshed way, or it may be simply goofy, or it may be inappropriately familiar. Certainly, his conduct is insubordinate and highly offensive to the colonel.

From Hubert Selby, Last Exit to Brooklyn (1957):

He [Spook] picked up a old police bike for few bucks and fixed it up. You know, threw some paint on it and stole a wildass buddyseat all covered with fur and chrome, and was fulla piss and vinegar ta go. We toldim ta play it cool and relax and celebrate Tommys marriage. But he flipped when someone tried ta get that goddamn hat off his head so we said OK, wed go downstairs and look at his bike. So we looked. Big deal. Yaknow, when the cops is finished with a bike, man, its had it. But it was a bike and it moved. I think that sonofabitch woulda used it even if he had ta push it or pedal it like a kiddy car. So he kicks it over after 5 minutes and we listen to it cough and miss and Spook went puttin off with a shiteatin grin on his face and we went back up stairs and a few minutes later he comes back. Smilin all over the goddamn place and the strap of his hat under his chin. I tellya man, it was a pissa.

This is may be the 1957 quotation that most sources have in mind when they date "shit-eating grin" to 1957, although it is not the only instance from that year (as noted bove). Still, the sense of the phrase aligns reasonably well with the definition in Dalzell & Victor: "a broad smile, ingratiating and unctuous"—with the "broad smile" part, anyway.

From Evan Hunter, Strangers When We Met (1958):

Altar was smiling, but the smile was a crud-eating one, and Larry's years in the service had taught him to recognize the crud-eating grin and to know what it usually preceded. He felt himself stiffening.

"Sit down," Altar said. "Want a drink?"

"No." Belatedly, he added. "Thanks."

"Well, sit down anyway."

"Sure." Larry sat in the easy chair to the left of the bar unit, facing the blue couch upon which Altar still sprawled.

"I don't like to do this," Altar said. "I don't like it when it's done to me, and Christ knows it's done often enough. But I think there's a difference here, and if I'm wrong I'll apologize. Can I talk to you like man, Larry? Or do I have to give you the schmaltz?"

Here, the "crud-eating grin" seems to be a kind of grimace of forced conviviality preceding the imparting of bad news (in this case, the bad news is that Altar thinks that Larry's rough sketches "stink").

From Thomas Williams, Town Burning (1959):

"...A man killed my dog, that's all I know! Daisy, she was the sweetest little dog you ever see. She never hurt nobody, except rabbits. She was all hell on rabbits. Shot forty rabbits front of Daisy one year. I damn near lived on rabbits that year. You ought to heard Daisy on a hot trail. Ki Yi Yi! Jesus! Wouldn't she go right out straight? Daisy'd dream about rabbits. Sometimes she'd wake right up out of her sleep a-running and a-yelling! Right after them rabbits, right out of her sleep! You'd see her little legs begin to twitching and her nose to puckering. Didn't she look ashamed to herself when she woke up! She'd look all around pretending they was a rabbit under my bed. I'd look down at her and grin, then she'd look up at me and give a shit-eating grin and go back to sleep. Damnedest dog."

The grin described here appears to far more sheepish than smug, since it involves being caught in a failed attempt at subterfuge.

From Richard Bissell, Good Bye, Ava (1960) [combined snippets]:

"Be quiet," I said. "You and your big mouth. You oughtta go with a good side show. 'Ryan and his crooked crew!' Why you are likely to end up riding out of town feet first."

"Maybe so, but I'll have company. I'll nail a few of them to the tan-board before I go. God damn it, Frank, somebody's got to do something. You don't care. Oh no. You just stand there with that shit-eating grin. Very funny. You don't care if they come with bulldozers, dredges, pile-drivers, tanks, machine guns! You just keep asmiling and agrinning like it was all a Sunday school picnic out at Riverside Park. By God, what would it take to wipe that foolish smile off your face?"

The character who introduces the expression "shit-faced grin" in this exchange goes on to refer to he other person's "foolish smile," suggesting that the grin is less smug than ill-suited to the occasion.

from an unidentified piece by Ivan Gold in The Noble Savage (1961):

The corporal has been back since, wearing a shit-eating grin and parrying attempts to get the whole story. Poor Pop has never returned. I, on my part, have come by a story which rounds out my repertoire, this tale of intrigue during the Fourth Moon ; it bristles, I think, with a peculiar significance. Don't you ... "

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A possible antecedent phrase: 'shingle-eating grin', 1949–1950

An earlier variant found in military periodicals is "shingle-eating"—which may invoke (for purposes of euphemism) the phrase "shit on a shingle," meaning creamed chipped beef on toast, a common entree during World War II.

From an unidentified article in The American Legion Magazine (1949):

No work of typewriter or camera is complete without The Homesick Kid from Kansas. When this lad appears at camp, he brings with him wide blue eyes, a butch haircut, a shy, shingle-eating grin, and a firm conviction that no real bullets will be used. The Kid wanders about telling the others how different Army life is from life on the farm in Kansas. He confides that he has joined up to fight for Ma's apple pie and an ice cream soda at the corner drug store. He fights in vain, however, since the pie is sent to Okinawa by mistake, and there are no corner drug stores where his battles are fought.

The main thing about the Kansas Kid is his inability to learn anything. He is the only person in the world who can go through a year of intensive training and tome out with his blue eyes as wide and blank as when he started. He never learns the simplest facts of military life, and is vague, dreamy, confused, stupid and homesick from the time he is drafted until he is knocked off.

The sense of "shingle-eating" as used in this excerpt is something like "bashfully stupid."

And from an unidentified piece in The Leatherneck (December 1950):

... Mason...! You O.K.?"

"Yeah, O.K. ... I guess," Herb yelled, looking himself over for the first time. "O.K.," he added again a big shingle-eating grin spreading over his face. The soft earth moved easily under him as he rolled over on his back . . . pleasant, sweet, fragrant. Inside his field shoes his toes wriggled gleefully. Through a newly made crack in the sandbags he could see the setting sun, no ordinary sun.

As in the previous example, the sense of the phrase here seems to be "goofily uncertain smile"—not smug, self-satisfied, or gloating.

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Conclusions

It seems clear from the slang dictionary definitions noted at the beginning of this answer that "shit-eating grin" has been applied to a fairly wide range of states of mind behind a grin so characterized: satisfaction, gloating, smugness, stupid gloating, self-satisfaction, ingratiation, unctuousness, slyness, duplicity, toadying, subservience. To these, on the basis of how the expression was used in the late 1950s, we might reasonably add shyness, bashfulness, awkwardness, foolishness, sheepishness, naïveté, forcedness, artificiality, false conviviality.

It is not at all clear to me that the earliest instances of "shit-eating grin" are intended to convey the sense of smug self-satisfaction that later lexicographers attribute to the phrase. The smug sense of "shit-eating grin" may have been a later development or it may simply represent one line of usage of an inconsistently understood expression, coexisting with such alternative senses as "sheepish grin," "awkward, self-conscious grin," and "uneasy or artificial forced grin."

How you assess the original expression's meaning depends to a great extent on the weight you give to "shingle-eating grin" as a possible antecedent or euphemistic early form of the phrase. If "shit-eating grin" emerged as G.I. slang during World War II and first appeared in print in the bowdlerized form "shingle-eating grin" in U.S. military publications, the two examples of "shingle-eating grin" cited above from 1949 and 1950 are highly relevant. The tenor of those two examples—especially the one from 1949—strongly suggests that the phrase may originally have applied to a grin that expressed shyness, naïveté, an awkward eagerness to please, and a kind of unguarded gullibility.

At least three instances of "shit-eating grin" from the late 1950s make sense when read in that way. Most notably, the Selby quote from 1957 (with the character's childlike delight in his surplus police motorcycle), the Williams quote from 1959 (with the dog's goofy attempt to validate or normalize in the waking world her excitement at chasing rabbits in her dreams), and the Bissell quote from 1960 (with the speaker's explicit characterization of the "shit-eating grin" with a "foolish smile") are thoroughly intelligible when read in this sense.

But the "eagerness to please" element of the Homesick Kid from Kansas's "shingle-eating grin" in 1949 might also have evolved into a contrary sense of "shit-eating grin"—one that emphasized toadying, smarmy unctuousness, and artifice. We see early instances of this line of meaning in the Hunter quotation from 1958 (in which the character with the "crud-eating grin" is trying to project conviviality in a forced manner) and (arguably) in the Gold quote from 1961 (where the grinning corporal parries all attempts to get the true story of some incident from him).

A third line of usage emphasizes smugness or gloating. The strongest instance of this usage seems to be the one in the Giovannitti quote from 1957 (in which an airman who loves to spread rumors relishes the moment of being in possession of a juicy story that his hearers are not already acquainted with); it may also be an element in the Gold quote, as artificiality and smugness are quite capable of shading into one another.

Today, the smugness line of meaning seems to be dominant, perhaps owing in part to the reinforcing influence of slang dictionary definitions. But as the Dalzell & Victor entry for "shit-eating grin" indicates, the ingratiating unctuousness and simple-minded broad grin senses of the term may still have some currency as well.