What is the origin of the idiom " rule of thumb" and when to use it?

The origin of the idiom "rule of the thumb" is not definitively known, but it is believed to have originated in the 17th century. It is believed to have derived from an old English law that permitted a man to beat his wife with a stick as long as it was no thicker than his thumb. This phrase has been in use since the 17th century, with one of the earliest recorded instances found in a book by Sir William Hope in 1692. It is used in various contexts and occasions where a practical, general guideline or estimate is needed.