The phrase "rule of thumb" is derived from an old English law which stated that you couldn't beat your wife with anything wider than your 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 seems that Buller was hard done by. He was notoriously harsh in his punishments, but there's no evidence that he ever made the ruling that he is infamous for. Edward Foss, in his authoritative work The The Judges of England, 1870, wrote that, despite a searching investigation, "no substantial evidence has been found that he ever expressed so ungallant an opinion". http://www.phrases.org.uk/meanings/rule-of-thumb.html