Myth: The popular expression 'rule of thumb' originated from English common law, which allowed a husband to beat his wife with a whip or stick no bigger in diameter than his thumb. The husband's prerogative was incorporated in to American law. Several states had statues that essentially allowed a man to beat his wife without interference from the courts.
It was true in some places (and unfortunately still is in too many) that a men were permitted to beat their wives, but this rule was never codified in English legal tradition; and a rule limiting such beatings to a stick of a particular size never existed. In fact, this story of the phase's origin does not appear until the 1970s. Nor are the claims backed up with actual citations of legal precedent.
The actual origin of the phrase, it is most likely an allusion to the fact that the the first joint of an adult thumb measures roughly one inch, quite literally a rule (or ruler) of thumb.The earliest citation of the phase rule of thumb in the Oxford English Dictionary is from Hope's Fencing-Master from 1692, centuries before anyone connected wife beating with thumb-sized sticks: "What he doth, he doth by rule of Thumb, and not by Art."
