An old English law declared that a man could not beat his wife with a stick any larger than the diameter of his thumb. Comment from Lance: When brewing beer, the temperature at which the yeast is added is important. Too cold and the yeast won't work, too hot and it'll die. Before the days of thermometers, a brewer would determine temperature by simply sticking his thumb into the mixture. Another possibility: As an artist, I always thought that this saying applied to the act of using one's thumb as a judge of scale. You literally use your thumb to act as a ruler to determine the scale of an object in your painting. Peter wrote: the French word for an inch (puce) is the same as for a thumb. I wonder if the thumb was used as a rough reckoner for inches? Just a thought. Anthony wrote: this has nothing to do with painting, nor is it anything to do with wife beating, which is just plain old down home American stupidity. It was used by bakers in judging flour, the flour being rubbed between thumb and fingers