from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A useful principle having wide application but not intended to be strictly accurate or reliable in every situation.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A general guideline, rather than a strict rule; an approximate measure or means of reckoning based on experience or common knowledge.
- n. Approximated, guesstimated.
from the GNU version of the Collaborative International Dictionary of English
- n. any rude process or operation, like that of using the thumb as a rule in measuring; hence, judgment and practical experience as distinguished from scientific knowledge.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a rule or principle that provides guidance to appropriate behavior
Michael Quinion lists the first documented use as 1692 while The Oxford English Dictionary puts the first documented use at 1685. Some suggested origins include the fact that the inch originated as the distance between the base of the thumbnail and the first joint, the practice of approximating the general direction of the wind by wetting the thumb then raising it in the air, and the rule of English Royal banquet plate setters using the distance of the thumb to equally space each plate from the table edge. Apocryphally, it has been claimed the term originally referred to the maximum thickness of a stick with which it was permissible for a man to beat his wife, but the earliest use in direct reference to domestic violence may be Del Martin’s 1976 book Battered Wives. (Wiktionary)