What is the origin of the term 'rule of thumb'?

The middle joint of the average man's thumb is usually about one inch (2.5cm) wide and was used by carpenters as a rough guide.

The Term "Rule of Thumb" also refers to an old Irish rule that stated a husband could beat his wife with a stick as long as it wasn't more round than his thumb. I had first heard this from the movie Boondock Saints and researched it a little more and found it to be true.

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.

http://www.worldwidewords.org/qa/qa-rul1.htm

The wife beating explanation for the expression 'rule of thumb' has pretty much been debunked.


The "rule of thumb," however, turns out to be an excellent example of what may be called a feminist fiction. It is not to be found in William Blackstone's treatise on English common law. On the contrary, British law since the 1700s and our American laws predating the Revolution prohibit wife beating, though there have been periods and places in which the prohibition was only indifferently enforced.

That the phrase did not even originate in legal practice could have been ascertained by any fact-checker who took the trouble to look it up in the Oxford English Dictionary, which notes that the term has been used metaphorically for at least three hundred years to refer to any method of measurement or technique of estimation derived from experience rather than science.

According to Canadian folklorist Philip Hiscock, "The real explanation of 'rule of thumb' is that it derives from wood workers ... who knew their trade so well they rarely or never fell back on the use of such things as rulers. Instead, they would measure things by, for example, the length of their thumbs."

https://wordsmith.org/board/ubbthreads.php?ubb=showflat&Number=152854

From Who Stole Feminism? - Christina Hoff
Sommers, (Simon & Schuster, New York 1994) (Excerpted from Ch 9 "Noble Lies" pp.203-208)

I heard it also comes from when people were working in a windmill grinding the corn

When they would set the two grinding stones at such a distance apart to get a certain consistency they would rub the ground corn between their thumb and finger, and it would be by "rule of thumb" if it was the correct consistency.

Or along those lines!

The precise origin is unknown, however, it is thought to have something to do with using the length or width of a thumb as a means of measurement. There are a few dubious sources that suggest that a husband could not beat his wife with a stick bigger than the width of his thumb which is disturbing.

Its earliest (1685) appearance in print comes from a posthumously published collection of sermons by Scottish preacher James Durham: "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."

Historically, the width of the thumb, or "thumb's breadth", was used as the equivalent of an inch in the cloth trade; similar expressions existed in Latin and French as well.[5][7] The thumb has also been used in brewing beer, to gauge the heat of the brewing vat.[2] Ebenezer Cobham Brewer writes that rule of thumb means a "rough measurement." He says that "Ladies often measure yard lengths by their thumb. Indeed, the expression 'sixteen nails make a yard' seems to point to the thumb-nail as a standard" and that "Countrymen always measure by their thumb."[9] According to Phrasefinder, "The phrase joins the whole nine yards as one that probably derives from some form of measurement but which is unlikely ever to be definitively pinned down.

https://en.wikipedia.org/wiki/Rule_of_thumb