rule of thumb

Etymology:

• Attested since the late 1600s. Of uncertain origin. One theory notes that the inch originated as the distance between the base of the thumbnail and the first joint, another notes the practice of approximating the general direction of the wind by wetting the thumb then raising it in the air. Another theory notes that English royal banquet plate setters used the distance of their thumbs to equally space each plate from the table edge.
• The erroneous claim that the term referred to the maximum thickness of a stick with which it was permissible for a man to beat his wife has appeared in the Washington Post and Time; it may originate from Del Martin's 1976 book Battered Wives.