Before going into the origin of this phrase, I should first refer to the popular folk etymology that claims that it originates from English law, supposed to have stated that a husband may beat his wife with a rod no thicker than a thumb, thus the “rule of thumb”. There has never been such a law, and there are no references whatsoever to it in any legal documents. Even with the much maligned Judge Sir Francis Buller, who was supposed to have issued such a ruling in 1792, there are no records of his ever making such a pronouncement, and the first suggestion that he did came a full year later in a cartoon.
However, the phrase “rule of thumb” long predates Buller’s alleged and elusive ruling.
We can find it in its present form from the 1600s. An early example in print, from 1685, is in Durham’s Heaven Upon Earth:
“Many profest Christians are like to foolish builders, who build by guess, and by rule of thumb.”
However, the phrase is much older than that, and is very likely linked to the use of the thumb as a unit of measurement. Indeed, in a number of languages, “thumb” has long been related to the word for “inch”, such as Italian pollice, which means both, Spanish pulgada, “inch”, which derives from pulgar, meaning “thumb”, or Slovenian palec, which also means both.
If we go back further in English history, to the ninth century and Old English, we can find a reference to this idea:
Swá greáte swá ðín þúma
(As large as a thumb)
This is from Leechdoms, Wortcunning, and Starcraft of early England, a collection of writings published in the 1800s, dating back to the late ninth century. Already at that time the thumb was being used as a means of comparing size, and it is no great leap from “as large as a thumb” to “the rule of thumb”.
Thumbs are conveniently attached to us and conveniently small – it seems natural that they should have been routinely used for measurement. And of course, as we have seen with “inch”, this is hardly restricted to English, and there are expressions with the same idea in other places too. Swedish, for example, has tumregel, also meaning “rule of thumb”, while Finnish uses a similar expression with nyrkkisääntö, which translates as “rule of fist”. German uses the same image, with Faustregel; “fist-rule”. Welsh uses synnwyr y bawd – “rule of thumb”.
Spanish does not use a thumb or hand related word, and instead prefers regla general, “general rule”, or also regla de oro, which translates as “golden rule”.
This brings me back onto German, and the expression Pi mal Daumen, or “Pi by the thumb”, a parody of a real calculation, if you like, for rough estimates or approximations. I do hope this is not a reflection of your maths lessons!
You can read about the etymology of thumb here.