Where does the phrase rule of thumb originate from? Why the thumb, of all possible body parts?
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.
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.
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.
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
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.
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").