The term "rule of thumb" or similar exists in many languages and cultures. Its likely origin is that the thumb is often used for rough measurement by carpenters, seamstresses, and many others.
A widespread myth holds that the term originally referred to the maximum size of a stick with which it was permissible for a man to beat his wife. This myth has been thoroughly debunked, for instance by Christina Hoff Sommers in her book Who Stole Feminism (1994 ISBN 0684801566). The myth is thought to have been invented by Del Martin of the National Organization for Women (NOW) in 1976 and continues to be spread to this day.