Why do we say ‘rule of thumb’?

What does 'rule of thumb' mean and why do we say it? Much like the phrase's meaning, establishing the origins of the idiom is a rough estimate...

A 1782 cartoon by James Gillray that depicts Buller as ‘Judge Thumb’. (Photo by Library of Congress)

Where does the phrase ‘rule of thumb’ come from? It frequently refers to using the tip of the thumb as a unit of measurement (which is as convenient as it is inexact).

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A commonly heard alternative, however, states the ‘rule of thumb’ was the creation of 18th-century English judge, Sir Francis Buller. He ruled (supposedly) that a man is legally permitted to beat his wife, provided he uses a stick no thicker than his thumb.

Buller was known for his overly harsh and prejudiced judgements, but there is no record of this repulsive ruling. At the time, men were permitted to admonish their wives in ‘moderation’, but the law was vague as to exactly how they could so and there was no mention of a ‘rule of thumb’.

Nevertheless, the theory against Buller was given credence by a 1782 cartoon by James Gillray. The illustration depicts Buller as ‘Judge Thumb’, carrying a couple of bundles of sticks, while a man chases a woman across the background wielding a rod above his head.

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This article was taken from BBC History Revealed magazine