Phrases.org.uk has some interesting discussion about the origin of this idiom. It has been incorrectly attributed to a supposed law in England in the 1700s that allowed a man to beat his wife as long as the stick was no thicker than his thumb. The origin is described as being unknown. However, it is speculated that thumbs have always been used to estimate distance by seeing how much of the object is covered by holding the thumb up and over the object. The thumb is also used to estimate the temperature of beer. The length of the thumb nail or the width of the thumb has also been used to estimate an inch.