Its the size of the stick that you could beat your wife with, hence the stick could be no larger than your thumb. Ergo, Rule of Thumb. I have a Rule of Dick, I beat my wife with it every night!
http://en.wikipedia.org/wiki/Rule_of_thumb the anti-women history of the phrase is up for debate, I've read from more than one source that the wife beating history is false and is merely an urban legend.
Thus the reason I mentioned the movie Boondock Saints, they get in a fight with a "large" woman over saying it. Although, I too have read this is an urban legend.
Who knows. My economics professor kept talking about it, so that's how I heard of it. He seemed old enough to have been around at the time, as well.
Is that what your Mom told you... I'm gonna have to pimp slap her for that 2 bucks she owes me! Respect!
