Even "The Bias-Free Word Finder," the bible of the Politically Correct Language Guardians among us, considers the wife-beating theory implausible and notes that it first surfaced in a 1986 letter to the editor in "Ms." magazine. So I guess the first "rule of thumb" in these cases is "Check your sources, lest they be hokum." - The Word Detective

According to the Discovery Science Channel's TV show, "Discoveries This Week" on 19 September 2005, the term comes from brewery industry before the advent of thermometers. The man in charge of aiding yeast would stick his thumb into the vat to check the temperature. - Wikipedia