“The 'rule of thumb' comes from a law allowing men to beat their wives with sticks no thicker than their thumb.”

The idea that the phrase "rule of thumb" originated from a law permitting men to beat their wives with sticks no thicker than their thumb is a pervasive misconception. In reality, the phrase dates back to the 17th century and refers to a practical method of estimation or measurement. Historically, the thumb was a convenient and readily available tool for making approximate measurements in various trades, such as brewing, carpentry, and textiles, where precision was not always paramount. The width of a thumb is roughly equivalent to an inch, making it a handy guide for quick assessments.

The connection to domestic violence emerged much later and is rooted in a misinterpretation of historical events. In the 18th century, a British judge named Sir Francis Buller was satirized in cartoons and rumors as "Judge Thumb" for allegedly stating that a man could beat his wife with a stick no thicker than his thumb. However, there is no official record of Buller ever making such a ruling. Despite this, the notion persisted, and in the 19th century, some American court decisions mistakenly referenced an "ancient doctrine" that allowed husbands to physically discipline their wives with implements no thicker than their thumbs. These rulings, though isolated and based on flawed understanding, contributed to the enduring folk etymology.

The myth gained significant traction in the 1970s when it was cited in some legal journals and reports on domestic abuse, leading many to believe its erroneous origin. The compelling, albeit false, narrative of a legal precedent for domestic violence made the myth particularly memorable and easy to believe, especially as societal awareness of domestic violence grew. This historical layering of rumors, misinterpretations, and later advocacy efforts cemented the false origin in popular belief.

Ultimately, the true origin of "rule of thumb" is far less sinister, stemming from practical, everyday measurements. The enduring myth serves as a fascinating example of how historical anecdotes and misunderstandings can shape our understanding of language.