Where do you think the phrase, ‘Rule of Thumb’ came from?
There are two thoughts here.
Brewmasters of old often tested the temperature of a batch of beer by dipping a thumb in the brew. Their long experience told them how well the beer was brewing. (I wonder if they were thumbsuckers as children?) One theory has it that our expression for a rough, guesswork estimate derives from this practice. My choice of brew is ice tea, hold the lemon and sugar.
Last Joint of the Thumb
More likely it stems from the ancient use of the last joint of the thumb as a measuring device for roughly one inch. I measured mine and it’s 1 1/4″. Have we grown longer thumbs throughout the generations?
Let me know if your thumb at the last joint to the tip is 1 inch or is it longer. And if you’re a brewmaster….how’s that thumb working out for you?
taken from the Henry Holt Encyclopedia of Word and Phrase Origins by Robert Hendrickson