where didthe phrase, "rule of thumb" come from? i know what its supposed to mean, but why?

Answer 1

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.

Answer 2

A means of estimation made according to a rough and ready practical rule, not based on science or exact measurement.

Origin

This 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. The following year James Gillray published a satirical cartoon attacking Buller and caricaturing him as 'Judge Thumb'. The cartoon shows Buller carrying two bundles of sticks and the caption reads "thumbsticks - for family correction: warranted lawful!"

It seems that Buller was hard done by. He was notoriously harsh in his punishments, but there's no evidence that he ever made the ruling that he is infamous for. Edward Foss, in his authoritative work The 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. Despite the phrase being in common use since the 17th century and appearing many thousands of times in print, there are no printed records that asspciate it with domestic violence until the 1970s. The false stories that assumed the wife-beating law to be true may have been influenced by Gillray's cartoon.

Even if people mistakenly believed that law to exist, there's no reason to connect the legal meaning with the phrase - which has been in circulation since at least 1692, when it appeared in print thus:

Sir W. Hope, Fencing-Master, 1692 - "What he doth, he doth by rule of Thumb, and not by Art."

That makes it clear that the origin 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 using the estimated inch from the joint to the nail, etc. It isn't clear which of these is the precise origin and this joins the whole nine yards as a phrase that probably derives from some form of measurement but which is unlikely ever to be definitively pinned down.

Answer 3

It comes from an old law that said it was legal for a man to beat his wife with anything that was narrower than his thumb. Or at least that's what I heard, I don't know for sure...

Answer 4

I guess from when the roman emperor decided when a gladiator lived or died by pointing with his thumb up or down.

Answer 5

http://www.phrases.org.uk/meanings/307000.html