The phrase "rule of thumb" is an idiomatic expression that refers to a widely accepted principle or guideline based on experience and practice rather than theory. It is not necessarily an exact science, but more of a general rule that people follow as a matter of habit or convenience. The origin of the term comes from the fact that it was once considered acceptable for men to beat their wives with a stick no thicker than their thumb, which shows how this phrase has evolved into something completely different from its original meaning. Today, "rule of thumb" is used in various fields such as science, engineering, and medicine, where it serves as a useful approximation or estimate that can help people make decisions without the need for complex calculations.