Origins: Rule of Thumb

Contrary to popular opinion, the origin of the above phrase is not that men in 18th century England were legally allowed to chastise their wives with a piece of wood no wider than the first joint of their thumb. That belief has now been exposed as a feminist hoax from 1976 which ultimately came to be universally accepted as accurate:

http://www.debunker.com/texts/ruleofthumb.html

The earliest citation of the phrase comes from Sir William Hope’s The Compleat Fencing-Master, second edition, 1692, page 157: "What he doth, he doth by rule of thumb, and not by art." The term is thought to originate with wood workers who used the length of their thumbs rather than rulers for measuring things, cementing its modern use as an inaccurate, but reliable and convenient standard.

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