Whenever talking about making a rough estimation because of an existing rule, the phrase for it, “rule of thumb,” is mentioned. I’ve said that I’ve wondered about the origin of this phrase. Every time that I say that my sister pipes up and says, “I know!” She then explains how it came about because there was an old rule where a man was allowed to beat his wife with anything that was as wide as his thumb, but no wider.
I’ll concede to her explanation, but I still wonder about it. It seems odd that one could make such a rule. After all, not everybody had the same width of a thumb. Was this based on the width of king’s thumb? Was there a certain measurement that was a “rule of thumb?” I’ve not heard of any such thing.
It turns out that this origins is completely spurious. It was said that Judge Sir Francis Buller made this rule in 1782. James Gillray even published a satirical cartoon that attacked Judge Buller.
The problem with this is that no law with this wording ever existed. British common law did have a law saying it was legal for a man to chastise his wife moderately, but there is nothing written out to tell anybody what that means.
The idea about “rule of thumb” coming from men being allowed to beat their wives is completely fictional and was just an idea that was spread by rumors among many for different reasons.
The phrase “rule of thumb” had been used about a hundred years earlier than Buller was judge, so it was in use before he was born.
It appears in print 90 years before Buller was judge. This is in The Complete Fencing Master which was Sir William Hope’s training manual for people who wanted to be swordsmen. He wrote, “What he doth, he doth by rule of Thumb, and not by Art.”
It is not completely certain where this phrase originated. However, it is likely that it refers to one or many ways that thumbs were used to estimate things. This could be how a thumb was held up to judge the distance of alignment of an object. Another idea is assuming the length of a thumb is about an inch and measuring things that way. A third idea is measure the temperatures of brews by sticking a thumb in them.
So, it seems that the origin of the phrase “rule of thumb” could itself be a rule of thumb.
Martin, Gary. “Rule of thumb.” The Phrase Finder. July 14, 2010. http://www.phrases.org.uk/meanings/rule-of-thumb.html>.