Well, it sounds plausible. “Rule of thumb,” some will tell you, referred to the English common law principle in colonial times that a husband was allowed to chastise (a polite phrase for “thrash”) his wife with a stick, so long as the stick was no thicker than a man’s thumb.

Trouble is, most of this isn’t true, according to impressive research set out by Prof. Henry Kelly in The Journal of Legal Education (1994). The phrase “rule of thumb” was indeed used in colonial times, that much is so. But its meaning then was the same as the common use today: a rule of thumb was a rough guide, based on experience or practice. Back to medieval times, the width of a man’s thumb has been used as a measure, equal to about one inch, so any man with a thumb had a rough rule (ruler) in hand. The claim that “rule of thumb” referred in colonial times to laws permitting wife-beating appears to be thoroughly modern invention by a journalist in the 1970s — now repeated so often that many people think it is true.

It is true that in colonial times, fathers and husbands were held responsible for the behavior of all the members of their household, and for that reason they were also allowed to exercise “moderate correction” “by domestic chastisement” over members in their household. There were even a few cases where husbands who were hauled before the courts for wife-beating argued in their own defense that the stick they had used was no thicker than their thumb. But they seldom got away with it, unless they happened upon a confused judge, because English law was direct on the point: “domestic chastisement” did not allow husbands to beat their wives. In one notorious case in 1782, where a British judge ruled that chastisement could include whipping with a thumb-size stick, he was skewered with public ridicule for his ignorance.

What is remarkable in Prof. Kelly’s account are the number of occasions when American courts in the 1800’s mistakenly referred to a supposed English principle that a man had a right to beat his wife with a stick no thicker than his thumb — but then the courts went on to rule that no such right existed under American law.

Curious that what survives in the popular imagination today is this odd myth that America’s Founding generation was a coarse and brutal bunch of wife-beaters.