In our episode on female astronauts, Molly and I perpetuated a myth during our listener mail segment. Say it ain't so!
We read an email about the supposed etymology of the phrase "rule of thumb," linking it to an old law about wife beating. As the myth goes, "rule of thumb" relates to a British law, allowing a husband to beat his wife with a stick, as long as it isn't wider than the man's thumb.
But Molly and I should've fact checked first. As a number of listeners have since written in to tell us, that "rule of thumb" theory is pure fiction. In English common law, "rule of thumb" was never the law of the land, either. WordOrigins found one judge who, in 1782, thought that husbands should be allowed to beat their wives -- and was publicly criticized as a result. The Straight Dope also offers a comprehensive explanation of how the "rule of thumb" roots became tangled, pointing out that while wife beating has never been legal in the United States, the 1976 NOW Task Force on Battered Women claimed that "the common-law doctrine had been modified to allow the husband 'the right to whip his wife, provided that he used a switch no bigger than his thumb'--a rule of thumb, so to speak." And from there, the misinterpretation of the phrase has persisted.
Listener Mat also cites linguist William Safire, who wrote that "The idea that rule of thumb is derived from an early form of spousal abuse is in error." Instead, the phrase probably came from carpenters who used their thumbs as a quick, handy measuring tool. From WordOrigins, "The phrase is almost certainly an allusion to the fact that the first joint an adult thumb measures roughly one inch, literally a rule (or ruler) of thumb."
Looks like we should also verify potential linguistic urban legends...as a rule of thumb.