It actually originated from a legal context in 17th century England.
* Back then, there were laws against wife beating, but the severity of the punishment varied.
* One such law stipulated that a husband couldn't be punished if his stick was no thicker than his thumb.
* So, "rule of thumb" literally referred to the thickness of a thumb as a legal limit.
Over time, this phrase took on a broader meaning. It started to refer to any rough-and-ready method of measurement or estimation, often used in everyday life, and sometimes with a hint of inaccuracy or imprecision.
Therefore, "rule of thumb" today signifies a practical, easily remembered guideline that is not always perfectly accurate but can be helpful in many situations.
It's a fascinating example of how language can evolve and take on new meanings over time!