What does “rule of thumb” mean, and is the wife-beating story true?

This cat-care video opens with: “The rule of thumb is to provide one litter box per cat plus one additional box.” A rule of thumb is a simple, practical guideline based on experience, not on strict measurement or science. It’s usually close enough to be useful, even if there are exceptions. Here, it gives a quick, easy number to remember: cats, plus one extra box.

Nobody is fully sure where the phrase started; it’s been in print since the seventeenth century and probably comes from using a thumb as a rough, handy measuring tool. A popular story claims it once meant a legal limit on how thick a stick a husband could use to beat his wife, but that story has been checked and shown false: no such law ever existed. The real history is much simpler: a thumb, used for quick estimates.