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Rule of Thumb


I’m back to inviting readers to submit quotations whose origins they want me to try to trace, using my book, The Yale Book of Quotations, and my more recent researches.
Joseph asked:

“Rule of thumb.  I have heard it was a common law rule about the thickness of a switch with which no punishment would occur for spousal abuse.  I have also heard that this is not correct.  I cannot find a definitive source and meaning.”

The popular etymology about “rule of thumb” originating with wife-beating is not correct. The Oxford English Dictionary states about “rule of thumb”:  “Probably so called on account of the thumb being used as a reference for approximate measurements of various kinds. … A suggestion that the phrase refers to an alleged rule allowing a husband to beat his wife with a stick the thickness of his thumb cannot be substantiated (compare the discussion by H. D. Kelly in Jrnl. for Legal Educ. 44 (1994) 341–65); it also poses semantic problems. The suggestion appears to be of late 20th-cent. origin, probably arising from a misunderstanding of the pun in the following passage (discussing the alleged rule mentioned above):  1976    D. Martin Battered Wives 31   [In 19th-cent. America] the common-law doctrine had been modified to allow the husband ‘the right to whip his wife, provided he used a switch no thicker than his thumb’ — a rule of thumb, so to speak.”
Do any readers have any other quotations whose origins they would like me to attempt to trace?