rule of thumb

The phrase is attested since 1658 and is of uncertain origin, though it probably derives from the use of the thumb as a convenient, if imprecise, instrument of measurement. One theory suggests it refers to the inch, which originated as the distance between the base of the thumbnail and the first joint. Another points to the practice of wetting a thumb and raising it to approximate the direction of the wind, while a third attributes the term to English royal banquet setters, who allegedly used their thumbs to ensure each plate was spaced equally from the edge of the table.

A persistent but erroneous claim suggests the term refers to the maximum thickness of a stick with which a man was legally permitted to beat his wife. This narrative appeared in both The Washington Post and Time magazine, though it lacks historical legal precedent. The story may have originated in Del Martin’s 1976 book, Battered Wives.