There is a fairly widespread belief that the term "Rule of Thumb" originally applied to an old English common law that stated a man could beat his wife, provided that any rod used was no thicker than his thumb. Over time "Rule of Thumb" then came to apply to any simple method of approximation.

A fair deal of investigation has gone into the veracity of this origin, and it appears that what we have here is simply another urban legend. The truth of the matter seems to involve the creative interpretation of some comments made by judges in obscure domestic abuse cases, and no codified law has been found describing what a husband should beat his sworn partner in life with. On the other hand, there IS a fair amount of information describing what the brothers and fathers of battered wives have done to abusive husbands throughout the years, and the bludgeons used in such cases were not restricted to any specific size.

The fact of the matter is there isn't any definitive etymology for the "Rule of Thumb" phrase. There are a number of references to carpentry, sewing, and bludgeoning implements, but no certain origin has actually been pinned down.

There is a fairly clear and in depth description of how the "Old English Law" myth arose at

I have also seen a number of references to an article titled "Rule of Thumb and the Folklaw of the Husbands Stick" by Henry Ansgar Kelly, published in the September 1994 edition of "The Journal of Legal Education". I was unable to track this article down personally.