The 'rule of thumb's' origin is generally put down to a law from a crap age gone by. It refers to the thickness of a 'weapon'(usually a stick or branch) that could be used legally to beat ones female partner with. If it was not as thick as 'the thumb' they used to measure the thickness it was fine, if it was thicker then it was naughty. Obviously men during that time who needed to know this law were ****ing ***kers ;)