Rule of thumb

Meaning

A practical guideline based on experience.

Usage

Used for general principles rather than strict laws.

Origin & History

Origins and dates here are approximate summaries. Many idioms have disputed or unknown origins, and not every date has been checked against a historical dictionary, so treat specific dates and stories as a starting point rather than a citation.

Probably from using the thumb as a rough measure. The popular story that it came from a law allowing husbands to beat their wives with a stick no thicker than a thumb is a myth; no such law existed.

First recorded in the 17th century.