The rule of thumb has been said to derive from the belief that English law allowed a man to beat his wife with a stick no thicker than his thumb. In 1782, Judge Sir Francis Buller is reported as having made this legal ruling and in the following year James Gillray published a satirical cartoon attacking Buller and caricaturing him as Judge Thumb. The cartoon shows a man beating a fleeing woman and Judge Buller carrying two bundles of sticks. The caption reads: thumbsticks for family correction warranted lawful. Wow – imagine if the phrase had been rule of arm! Ouch!
It seems that Buller stands falsely accused. He was notoriously harsh in his punishments and had a reputation for arrogance, but there's no evidence that he ever made the infamous ruling. Edward Foss, in his authoritative work 'The Judges of England (1870)' wrote that, despite a searching investigation, "no substantial evidence has been found that he ever expressed so ungallant an opinion". It's certainly the case that, although British common law once held that it was legal for a man to chastise his wife in moderation (whatever that may mean), the 'rule of thumb' has never been the law in England.
The fabled notion (wife-beating by legal precedence) was castigated by feminists in the 1970s. What is not clear is if this was due to Judge Thumb Buller and/or Gillray’s cartoon, or The Rolling Stones' song Under My Thumb that was released in 1966.
In reality the origin of the phrase remains obscure. It is likely that it refers to one of the numerous ways the thumb has been used to measure things; the alignment or distance of an object by holding the thumb in one's eye-line, the temperature of brewing beer, an ‘inch’ (the distance from the thumb joint to its tip), and the like.
The phrase joins the whole nine yards of the many ways in which things can be measured without being specific to any one of them. To this day, Afrikaans calls an inch ‘n duim (literal translation of ‘a thumb’).
Courtesy of South Africans in Portugal.