When “the rule of thumb” is used today, it means something that is measured by experience or an educated guess rather than something that is founded on science. However, looking at the origin of the phrase, a person quickly comes up against an unfounded and incorrect myth.
The phrase was used at least as long ago as the 1600’s. At the time, it was legal for a man to beat his wife, provided that he didn’t use excessive force. That doesn’t make much sense today, since beating a wife would be to use excessive force, even if it was a slap.
However, the courts of England, and later America, could and did defend a husband’s right to hit his wife. It wasn’t widespread and the practice was questioned even back then, but this is the origin of the myth, though not of the phrase.
As the myth goes, British Common Law allowed a man to strike his wife with a stick, provided that the stick was no thicker than the man’s thumb. Hence, it was supposedly the ‘rule of thumb’.
Thankfully, this is an untrue myth. Although there were cases of men being acquitted in court for beating their wives, which was admittedly a terrible thing, this had nothing to do with the phrase. It is just as likely that men were found guilty of the same offense. It still had nothing to do with the saying. In fact, such an origin wouldn’t even come close to the current use and meaning. Indeed, wife beating aside, it was never a law that a man could strike his wife with a stick as long as it wasn’t thicker than his thumb. That was totally made up.
The saying is so old that the actual origin may never be known, but it is likely that the origin was far more innocent, though still a bit disgusting.
You see, back then, beer brewers were attentive to the temperature of a vat of beer. For the best beer and best results, a certain temperature needed to be maintained. To test the temperature of the beer, many beer makers would simply periodically stick their thumb into the beer. This was an approximation and it certainly wasn’t scientifically accurate, but it apparently worked. It was the rule of thumb.
Sometimes the folklore surrounding the origin of phrases is far more colorful than the actual origin. This is one such case. The meaning is still the same, too. It simply has been expanded to have a meaning in more than just beer making.