Rule of thumb

A rough, practical guideline based on experience rather than exact measurement or science, offering a handy but imperfect way to judge or decide. It implies a shortcut-reliable enough yet approximate.

From 17th-century England, 'thumb' as a measure came from artisans using their thumbs' width (about an inch) to gauge roughly-mentioned in a 1692 fencing manual. A darker myth ties it to a supposed law letting men beat wives with sticks no thicker than a thumb, but no such law existed; it's a 20th-century misreading. The real root is mundane craft, growing into a proverb by the 1800s for seasoned guesswork.