LawProse Lesson #228: Is “rule of thumb” offensive?
A rule of thumb is “a roughly practical measure that is neither precise nor invariable.” The term almost certainly derives from the habit of tailors’ or carpenters’ use of the thumb as the rough measurement of an inch. The earliest known use of the term dates from 1685: “Many profest Christians are like to foolish builders, who guess, and by rule of thumb.” (J. Durham, Heaven upon Earth). An urban legend dating from the 1970s claims that the phrase is offensive because it originally denoted the width of a rod with which English law allowed husbands to beat their wives. It gained enough credence that in 1982 the U.S. Commission on Civil Rights titled a report on battered women, “Under the Rule of Thumb.” If that old legend were historically accurate, of course, the phrase would be repugnant in the extreme. But it is a bogus claim whose evidence postdates the origin of the phrase by at least a century. The best account of the debunking of the legend is found in James E. Clapp et al., Lawtalk: The Unknown Stories Behind Familiar Legal Expressions 219–25 (2011). Further reading: Black’s Law Dictionary 1533 (10th ed. 2014).