Beer Trivia 1
A "rule of thumb" means a rough or guesswork estimate, based more upon experience than on precise measurement.There are two theories about the origin of this expression. One theory is that it comes from the frequent use of the lower part of the thumb (roughly equal to one inch in the average adult male) as a crude measuring device. However, some authorities trace the phrase to a practice once common among brewmasters. In the days when beer was truly beer, not the pasteurized soft drink that passes for beer today, the chief brewer sometimes tested the temperature of a batch of brew by dipping in his thumb. This technique was neither so accurate nor so hygienic as a thermometer check would be, but based on the brewmaster's long experience, this 'rule of thumb' would tell him how well the brewing was proceeding." From the "Morris Dictionary of Word and Phrase Origins" by William and Mary Morris (HarperCollins, New York, 1977, 1988). Page 500-501.