** Phrases: Rule of Thumb

Meaning:

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. That same year James Gillray published a satirical cartoon attacking Buller and caricaturing him as 'Judge Thumb'.

It's quite possible that Buller was hard done by. He was notoriously harsh in his punishments but there's no clear evidence that he ever made the ruling that he is infamous for.

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.

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 eyeline, 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.