The Rule of THUMB is an old English common law principle that states that a person may use reasonable force in self-defense, so long as the force used is not excessive. This principle has been used to determine the legality of certain actions in court cases. The phrase 'Rule of THUMB' is believed to have originated from the idea that a man was allowed to beat his wife with a stick no thicker than his THUMB. This phrase has since been used to refer to any general principle or guideline.