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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



And the Discovery Science Channel is a fount of etymological wisdom.