For centuries rule of thumb has been used as an estimated measurement by many craftsmen including carpenters, brewers, and tailors. The term referred generally to the length of the thumb from the joint to the tip, approximately an inch. It was once quite common to use body parts as measurement references (such as a hand in measuring horses) and some of the reference endure today in one form or another.
Common Misconceptions About Rule of ThumbUnfortunately, many sources have pounced on another origin that says the term refers to the size of a stick that a husband could use to beat his wife. While the term may indeed have cropped up centuries later in reference to wife-beating, the term predates that obscure usage. At this site, rule of thumb is used as described in the definition above.
Rule of Thumb: Not Really - Another Myth of Women's History - About Women's History Guide Jone Johnson Lewis explains the common misconception about rule of thumb being a reference to wife beating. The article deals primarily with the very real issue of domestic violence and its linking to the term rule of thumb.
The Rule of Thumb for Wife-Beating Hoax - Debunker.com quotes many sources with discussion of where the misconception regarding domestic violence originated, the legal and casual references, the Oxford English Dictionary definition, and other historical references to rule of thumb as an approximation having nothing to do with wife-beating.