Where did the phrase "As a rule of thumb." come from?

Answer 1

NeedsHelp stated what many people nowadays THINK was the "rule of thumb".

Afraid not.

In fact, there is no evidence of this claim until VERY recently, in the 1970s!! Unfortunately, at that time, unwitting feminists who heard the claim were quick to spread it. Now I'm not saying they made it up, but apparently the IDEA fit so well with their expectations that they just ASSUMED it was true. But they never came up with the evidence.

Several folks have done quite a bit of study of English common law to see if there is any support for the claim. There really isn't.

In fact, "RULE of thumb" does not originally refer to a law at all. Rather "rule" here is used in the sense of "ruler", that is, "measuring device." The thumb has at times been used for approximate measure of an inch (cf. use of foot and forearm to measure things). The whole idea of the expression is something that's a good, rough approximation. When you use something as a "rule of thumb" you treat it as something that is generally true or fairly close.


The following article contains more information on the alleged connection of this expression with 'permissible wife beating', showing what little foundation that notion has. For starters, Blackstone's writings never say any such thing! (And notice that, when you read this claim, even when they say that it's in such-and-such a work, they somehow NEVER cite chapter and verse!)
http://www.wordorigins.org/index.php/more/489/

This article cites Blackstone's Commentaries on the Laws of England (1765), in which he mentions that at ONE time (but NOT in Blackstone's own day) a man was permitted to five his wife “modest punishment". It goes on to quote the entire passage in which this appears ... and there is NO mention of the thumb at all!!

One other problem with the legend. The explanation given above (use of the thumb for a rough measure) fits well with the way we use the expression. But the legend does not explain how someone got from a supposed limitation on how thick a stick was allowed to be to the meaning 'general, rough principle'.

Answer 2

"As a rule of thumb"... The colloquial phrase "rule of thumb" is supposedly derived from the ancient right of a husband to discipline his wife with a rod "no thicker than his thumb." (Time magazine, September 5, 1983)

A husband's right to beat his wife is included in Blackstone's 1768 codification of the common law. Husbands had the right to "physically chastise" an errant wife so long as the stick was no bigger than their thumb - the so-called "rule of thumb." (Washington Post, January 3, 1989)

HOWEVER!!!!

According to Canadian folklorist Philip Hiscock, "The real explanation of 'rule of thumb' is that it derives from wood workers ... who knew their trade so well they rarely or never fell back on the use of such things as rulers. Instead, they would measure things by, for example, the length of their thumbs."

Hope I helped!