Origin of the "Rule of thumb" phrase

Perhaps it's because the length of the thumb joint to the end of the thumb is a fairly accurate representation of an inch. So rule of thumb was likely a way to quickly verify the measurement before cutting for construction work rather than search for a yard stick. The meaning probably began to be used in a more abstract sense as a rule to quickly validate something.

No one knows. The expression has existed in many languages for a long time, which suggests that its origin is pretty old.

There are several theories, some based in the similarities in many languages between the words inch and thumb and how you can measure an inch using the thumb, others based on the general usefulness of the thumb to measure different things.

It's entirely possible that it originally had nothing at all to do with the thumb; that it was a similar word that has become distorted over time, then translated to other languages in its distorted form.

See also: Rule of thumb on Wikipedia.

If you ever do any carpentry you can get pretty close with your thumb as a rule / gauge.

So any measurement not using actual instruments is by "rule of thumb", or "by eye".

Rule of Thumb is an undocumented, implied rule to abide.

One possible origin, or at least implementation, is found in the 117th New Constitution of Roman Emperor Justinian I, published in 529 C.E., granting a husband freedom to "beat his wife with a whip or rod" for divorcable offenses. The "rule of thumb" was possibly conjured to suppress nefarious abuse of the law. This rule has never been documented, as it is merely a guideline, to help prevent serious injury.

An example rule of thumb is: "Slice a peach along its suture, to remove the pit." There's no harm in bisecting the fruit along any other geodesic. It's simply easiest to remove the pit when bisected in a specific manner. It's also a good rule to not call the suture of the peach its "butt crack" (or worse), as this will likely result in a negative impact to social status among your peers.

The failures of man to abide by rules of thumb resulted in the invention of posting warning signs for the most unlikely and oddly specific circumstances.

---

UPDATE:

As @NathanCTresh highlighted in his comment, the rule of thumb is indeed an oral tradition. Because this is rule passed on in oral tradition, the term is spoken in modern English ("rule of thumb"), and not in Latin ("pollux regula").

Not sure if it helps, but this expression reminds me of basic Physics lessons in secondary school: Fleming's "rule of thumb"/right hand rule (similar Oersted's rule in the wiki article posted by Guffa)

Prediction of direction of field (B), given that the current I flows in the direction of the thumb

(via wikipedia)

The origin is most likely derived from the use of the thumb as a practical way to measure/estimate different things such as distances or temperatures:

From the Phrase Finder :

• The phrase itself has been in circulation since the 1600s. The earliest known use of it in print appears in a sermon given by the English puritan James Durham and printed in Heaven Upon Earth, 1685:

  • "many profest Christians are like to foolish builders, who build by guess, and by rule of thumb, (as we use to speak) and not by Square and Rule."

• The origin of the phrase remains unknown. It is likely that it refers to one of the numerous ways that thumbs have been used to estimate things - judging the alignment or distance of an object by holding the thumb in one's eye-line, the temperature of brews of beer, measurement of an inch from the joint to the nail to the tip, or across the thumb, etc. […]

The earliest instance unearthed by Gary Martin (Phrasefinder) is in a rhyme found in Augmented with Ingenious Conceites for the Wittie and Merrie Medicines for the Melancholic, published in 1640:

If Hercules tall stature might be guess'd
But by his thumb, the index of the rest,
In due proportion, the best rule that I
Would chuse, to measure Venus beauty by,
Should be her leg and foot:

According to World Wide Word the origin is more likely to derive from measuring distances rather than temperatures:

• The expression rule of thumb has been recorded since 1692 and probably wasn’t new then. It meant then what it means now — some method or procedure that comes from practice or experience, without any formal basis. Some have tried to link it with brewing; in the days before thermometers, brewers were said to have gauged the temperature of the fermenting liquor with the thumb (just as mothers for generations have tested the temperature of the baby’s bath water with their elbows). This seems unlikely, as the thumb is not that sensitive and the range of temperatures for fermentation between too cool and too warm is quite small.

• It is much more likely that it comes from the ancient use of bits of the body to make measurements. There were once many of these: the unit of the foot comes from pacing out dimensions; the distance from the tip of the nose to the outstretched fingers is about one yard; horse heights are still measured in hands (the width of the palm and closed thumb, now fixed at four inches); and so on.

• There was an old tailors’ axiom that “twice around the thumb is once around the wrist”, which turns up in Gulliver’s Travels. It’s most likely that the saying comes from the length of the first joint of the thumb, which is about an inch (I remember once seeing a carpenter actually make a rough measurement this way). So the phrase rule of thumb uses the word rule in the sense of ruler, not regulation, and directly refers to this method of measurement.

and also from here:

• The real explanation of 'rule-of-thumb' is that is derives from wood workers (or other constructors) who knew their trade so well they rarely or never fell back on the use of such things as rulers. Instead they'd measure things by, for example, the length of their thumb; they measured, not by a rule(r) of wood, but by rule of thumb. The term was already in metaphorical use by the late 17th century.

The Phrase Finder also considers the possible origin that refers to the "legal" rule of beating with a stick one's wife, but not enough reliable evidence has ever been found to support it:

• The 'rule of thumb' has been said to derive from the belief that English law allowed a man to beat his wife with a stick so long as it is was no thicker than his thumb. In 1782, Judge Sir Francis Buller is reported as having made this legal ruling and in the following year James Gillray published a satirical cartoon attacking Buller and caricaturing him as 'Judge Thumb'. The cartoon shows a man beating a fleeing woman and Buller carrying two bundles of sticks. The caption reads "thumbsticks - for family correction: warranted lawful!"

• […]but there's no evidence that he ever made the ruling that he is infamous for. Edward Foss, in his authoritative work The Judges of England, 1870, wrote that, despite a searching investigation, "no substantial evidence has been found that he ever expressed so ungallant an opinion".

• It's certainly the case that, although British common law once held that it was legal for a man to chastise his wife in moderation (whatever that meant), the 'rule of thumb' has never been the law in England.

• […] there are no printed records that associate it with domestic violence until the 1970s, when the notion was castigated by feminists. The responses that circulated then, which assumed the wife-beating law to be true, may have been influenced by Gillray's cartoon or were possibly a reaction to The Rolling Stones' song 'Under My Thumb', which was recorded in 1966.

Rule of thumb A very old sailor (sailing ships,I am now 72 years old) I knew as a kid always said rule of thumb was a quick system of measurement on sea charts .... a thumb’s width on any chart, regardless of scale, assured a “safe” or heaithy distance off shore. Padken

Early English Books Online reports three instances of the expression from before 1700. They are as follows.

From John Alexander, Jesuitico-Quakerism Examined, or, A Confutation of the Blasphemous and Unreasonable Principles of the Quakers with a Vindication of the Church of God in Britain, from Their Malicious Clamours, and Slanderous Aspersions (1680):

That the Scriptures are Infallible is sufficiently proved at the Survey of their former Query; where we proved them to be of Divine Inspiration, and the Word of the Living God, and not of any meer Creature, and so they cannot deceive; so that for their Infallibility we need say no more. That therefore the Scriptures are the Rule of Faith and Manners I prove.

For first, That must be the Rule of Faith and Manners by which every matter of Faith and Manners ought to be examin'd seeing every thing that is examined must be examined by its Rule, or else it will be done by Guess and Rule of Thumb, as the Jest is. But every matter of Faith and Manners ought to be examined by the Scriptures: Ergo the Scriptures are the Rule of Faith and Manners. The Major is proved already. I prove the Minor from Isai. 8.20. Where we are expresly Commanded to go to the Law and the Testimony with every matter of Faith and Manners, and it is positively declared, that if it be not according to these, there is no Light in it, which is a sufficient Prohibition to receive it. And the Law and Testimony are the Scriptures. Exod. 32.15. and 34.29. Deut. 31.24, 26. 2 King. 22.8. Neh. 8.1, 3, 8. Psal. 78.5.

This instance suggests that there was a jest involving a "Rule of Thumb" widely enough known in 1680 not to require full retelling to be recognized by the author's audience.

From James Durham, Heaven upon Earth in the Serene Tranquillity and Calm Composure, in the Sweet Peace and Solid Joy of a Good Conscience Sprinkled with the Blood of Jesus and Exercised always to Be Void of Offence Toward God and Toward Men (1685):

Because there is much false bulk, and empty shew of a profession; Which we are afraid, but a very little tryal will quickly evidence, and discover to be so, even to be unsure work; I will not say, but there may be some sickernesse, and solidity in the profession of some, but it is to be feared, that many others are but building castles in the air, castles of come down when the rain shall descend, the winds blow, and the floods beat, having much more shew then substance, and solid work; and the way to make it sicker, sure and solid work, that will abide the tryal, is to lay it to the Rule, and to try it thereby; many profest Christians are like to foolish builders, who build by guess, and by rule of thumb, (as we use to speak) and not by Square and Rule; and if they have but bulk enough, they look not much to the solidity, and straightness of the Fabrick: There are many engadgements come under, many fastings, prayers, hearings of the Word, &c. (which are good in themselves, and commendable) but there would be Self-examination, to see what solidity is in them all; else we will be like the men, who expend all their stock on the stone and timber-work of a large house, and leave nothing to plenish, and furnish it within; whereas, when a person is suitably, and seriously taken up with Self-examination, it maketh a house (to speak so) though it should be lesse, to be well filled and furnished; and albeit there be no great noise of a Profession there, yet there is more kindly Repentance, more seriousnesse in Prayer, and in other Duties, more profiting by hearing of the Word, and more holinesse of Life, then where this exercise of Self-examination, and reflection is either altogether or very much neglected, albeit there should be then, a far greater profession.

This is the instance that Phrase Finder identifies as the earliest occurrence of "rule of thumb" according to user66974's answer.

And from William Hope, The Compleat Fencing-Master in Which Is Fully Described the Whole Guards, Parades & Lessons Belonging to the Small-Sword : As also the Best Rules for Playing Against Either Artists or Ignorants with Blunts or Sharps (1691):

I think you may easily apply this Story to what I was before saying, That a Man may break his Adversaries Measure, and that as often as he thinketh it convenient for his own safety, without being any wayes accounted a Coward. I know very well that those who understand this Art will be of my opinion, because they know that the Judging of Distance exactly is one of the hardest things to be acquired in all the Art of the smal-Sword; and when once it is acquired it is one of the usefulest things, and sheweth a Mans Art as much as any Lesson in it; but I am for no Mans Retiring too much, unless upon a very good Design, and that hardly any Ignorant of this Art can have, because what he doth (as the common Proverb is) he doth by rule of Thumb, and not by Art.

This example is interesting because it describes doing something "by rule of Thumb and not by Art" as a proverb. Nevertheless, the earliest instance I've been able to find of of "rule of thumb" treated as a proverb is in James Kelly, A Complete Collection of Scotish Proverbs: Explained and Made Intelligible to the English (1721):

No Rule so good as Rule of Thumb, if it hit.

[Comment:] But it seldom hits! Spoken when a Thing falls out to be right, which we did at a Venture.

At least in its early Scottish incarnation, the "rule of thumb" appears to have been an acknowledgment of lucky guesswork.

---

Conclusions

The phrase "rule of thumb" in the sense of "appraisal by guess or (perhaps) by reference to personal experience" was in use in England by 1680 and, more particularly, was associated by that date with an unidentified jest about such guesses. Authors in the 1680s present this source of appraisal or measurement in unflattering contrast to the objective and more fundamentally sound rule of Scripture or of "Square and Rule." A subsequent instance of the expression from 1691 contrasts "rule of Thumb" with "Art."

Finally, in Scotland by 1721 "rule of thumb" seems to have been used in connection with guesswork that came out well, in a formulation that acknowledged the overall unreliability of such guesswork.