The Word Nerd’s rule of thumb: don’t believe folk etymologies

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I’d been getting together weekly with a group of friends for around a year where we indulge in some banter whilst scoffing croissants and imbibing varieties of java.

We originally called these meetings “The Summit” but after several months of not coming close to solving any world problems and owning up to the mundane nature of our discussions we re-dubbed it “the swamp.” In any case, one time during one of our rare erudite sessions, we must have discussed some language issue, because a lady came over to our table and said she was listening to our language bavardage and asked whether we knew that the origin of the expression “rule of thumb” came about to dictate the legal length of an object a man could use in order to beat his wife.

Mind you, this myth is oft repeated. Take the following explanation found in Women: A Feminist Perspective, edited by Jo Freeman: “The popular expression ‘rule of thumb’ originated from English common law, which allowed a husband to beat his wife with a whip or stick no bigger in diameter than his thumb. The husband’s prerogative was incorporated into American law. Several states had statutes that essentially allowed a man to beat his wife without interference from the courts.”

In the 1980s, Time magazine wrote, “The colloquial phrase ‘rule of thumb’ is supposedly derived from the ancient right of a husband to discipline his wife with a rod ‘no thicker than his thumb,’ ” and in 1989 Washington Post added, “A husband’s right to beat his wife is included in the 1768 codification of the common law. Husbands had the right to ‘physically chastise’ an errant wife so long as the stick was no bigger than their thumb – the so-called ‘rule of thumb’”.

Actually, nobody has been able to find a single English or American law that ordains this conjugal thumb right to a husband.  It has been claimed that in 1782 British judge Sir Francis Buller proclaimed that a husband may beat his wife with a stick not thicker than his thumb but nobody has been able to discover documentation of such. On the contrary, 18th century British and American law  prohibit wife beating (though often this provision was only casually enforced.)

That the phrase did not originate in legal practice is verified by the “rule of thumb” entry  in the OED: “A method or procedure derived entirely from practice or experience, without any basis in scientific knowledge; a roughly practical method. Also, a particular stated rule that is based on practice or experience.” The first citation is from 1658: “Many profest Christians are like foolish builders who build by guess and by rule of thumb.”

The expression probably comes from the world of wood-working where ancient practitioners would rarely use rulers but would measure things by the length of their thumbs. It’s most likely that the saying comes from the length of the first joint of the thumb, which measures approximately one inch. An alternate theory, posited by other etymologists, credits the origin with brewmasters who often tested the temperature of the beer (before the invention of the thermometer, by dipping a thumb in the brew. This seems unlikely to me as the thumb is not that sensitive and the fermentation range between too warm and too cool is not appreciable.

In a subsequent column, I’ll look at some other folk etymologies.

Richler’s latest book is Wordplay: Arranged and Deranged Wit

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