Origin of the phrase....

Rule of Thumb - This has been said to derive from the belief that English law allowed a man to beat his wife with a stick so long as it is was no thicker than his thumb. In 1782 Judge Sir Francis Buller is reported as having made this legal ruling.

It's certainly the case that, although British common law once held that it was legal for a man to chastise his wife in moderation (whatever that meant), the 'rule of thumb' has never been the law in England.

This is a good site to find the meanings and origins of other well known phrases.