As a would-be dispenser of practical advice, I have often used the expression “rule of thumb.” Some feminists have argued that the phrase should be dropped, though, because it referred to a rule of English common law that a man could beat his wife if he did not use a stick thicker than his thumb. For example, former National Organization for Women president Patricia Ireland asserts this as fact in the widely seen PBS documentary, “Marriage: Is It More Than a Piece of Paper?”
I am happy to report that this is an inaccurate folk etymology. Indeed, even its appearance in discussions of English common law treat it as a folk belief which was not validated by the law. The one judge who is said to have articulated that standard from the bench may not even have uttered any sentiment so “ungentlemanly.”
The notion that rule of thumb is a wife-beating “fact” seems to have become widely reported only in the 1970s – the very time that Patricia Ireland probably heard it.