Stanford’s loonie, we know that

But they’re also wrong:

Even “rule of thumb” appears to have fallen foul of the university’s initiative, because it refers to “an old British law that allowed men to beat their wives with sticks no wider than their thumb”.

Sounds remarkably unlikely. Partly because there is no such thing as “British law” and never has been. The constituent Kingdoms have always had their own laws. But rather more to the point, back when we could have had such a law we didn’t have such laws. We can imagine (no, go on) that back in that past it was considered OK for some mild and unconsensual chastisement to go on. But back then we never did have laws which detailed what you may do. We only had laws which said what you may not do. That was just the way the English (and Scots etc) legal systems worked. Everything was legal except that which is illegal. Just the way the system rolled.

So we couldn’t have had a law which said that you were allowed to do so.

Quite apart from all the other reasons that the derivation is bollocks of course. Rule of thumb coming from the same source as the measurement the inch – a rough guide to something, around and about, as measured by the size of a thumb – the inch.

One of those things where Wikipedia is in fact correct:

In English, the phrase rule of thumb refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associated with various trades where quantities were measured by comparison to the width or length of a thumb.

A modern folk etymology holds that the phrase is derived from the maximum width of a stick allowed for wife-beating under English common law, but no such law ever existed. This belief may have originated in a rumored statement by 18th-century judge Sir Francis Buller that a man may beat his wife with a stick no wider than his thumb. The rumor produced numerous jokes and satirical cartoons at Buller’s expense, but there is no record that he made such a statement.

English jurist Sir William Blackstone wrote in his Commentaries on the Laws of England of an “old law” that once allowed “moderate” beatings by husbands, but he did not mention thumbs or any specific implements. Wife-beating has been officially outlawed for centuries in England (and the rest of the United Kingdom) and the United States, but continued in practice; several 19th-century American court rulings referred to an “ancient doctrine” that the judges believed had allowed husbands to physically punish their wives using implements no thicker than their thumbs.

The phrase rule of thumb first became associated with domestic abuse in the 1970s, after which the spurious legal definition was cited as factual in a number of law journals, and the United States Commission on Civil Rights published a report on domestic abuse titled “Under the Rule of Thumb” in 1982. Some efforts were made to discourage the phrase, which was seen as taboo owing to this false origin. During the 1990s, several authors correctly identified the spurious etymology; however, the connection to domestic violence was cited in some legal sources even into the early 2000s.