A common, but erroneous, belief circulating among feminist writers is that the phrase "rule of thumb" derives from an archaic, oppressive, patriarchal custom or legal ruling that a husband may beat his wife freely so long as the stick is no greater in circumference than his thumb.

Actually, if you consult Brewer’s Dictionary of Phrase and Fable (a trove of interesting facts), you will find that "rule of thumb" means "by rough approximation," deriving from the worker’s custom of using the thumb for measurements: "The first joint of the adult thumb measures almost exactly 1 in (2.5cm)."

The misconception of the origin of the term is an example of folk etymology, by which popular lore ascribes meanings and origins to words and phrases.

Another example is "Welsh rarebit," which some mistakenly consider to be the original term for what is now called "Welsh rabbit." On the contrary, "Welsh rabbit" is the original and correct term, indicating that the cheese-and-milk-on-toast dish is a substitute for meat. Brewer’s entry mentions "mock turtle soup" and "Bombay duck" (a dried fish with curry) as comparable terms.

Folk etymologies develop to give people a sense of context and historical understanding. One clue in identifying them is that they tend to fit meaning and origin too neatly together. When a high school classmate solemnly informed me that the most popular Anglo-Saxon monosyllable is an acronym for Fornication Under Consent of the King, I knew that something about that could not be right. A dictionary acquainted me with the Middle Dutch word fokken, and all was plain.