Rule of thumb
Tim
Art or architecture / building would seem to be the obvious sources.

Any thoughts?

chpwldmn
Long ago the whith of a man's thumb was used to measure one inch or so I have been told.It is also about 2 centimeters. Tell me if I'm wrong.
Frank Pierce
It means working on a general hunch or intuition, where exactitude isn't really required. I would guess that "rule" has to do more with the word "ruler" as a carpenter's measuring device than it does with requirement, as in rule of law.

The thumb makes a good general measuring device for reasonably small quantitities.

Lewis Joplin II
RULE OF THUMB - "There are two good choices here. Brewmasters of old often tested the temperature of a batch of beer by dipping a thumb in the brew, their long experience telling them how well the beer was brewing. One theory has it that our expression for a rough, guesswork estimate derives from this practice. More likely it stems from the ancient use of the last joint of the thumb as a measuring device for roughly one inch." From the "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Facts on File, New York, 1997).

A second reference also lists these two theories and adds a couple of details. The lower part of the thumb is roughly one inch long in the average adult male. And what the brewmaster was checking was the temperature of the beer, a practice that was "neither so accurate nor so hygienic as a thermometer check..." From the "Morris Dictionary of Word and Phrase Origins" by William and Mary Morris (HarperCollins, New York, 1977, 1988).

Frank Pierce
Lewis, I give the old Brewmeister my vote. It's quite possible to guess with amazing accuracy the termperature of a liquid with your fingers. Having worked with darkroom developing for years, I found I could guess the temperature of a liquid within a degree F. just by feel.

On the sanitation business, I agree. I never tried to drink my Dektol.

AtoZ
I have ABSOLUTELY no documentary evidence for this, but I have convinced myself (and that's a start) that the REAL origin of this phrase is somewhat different from the above. It's a widely used practice to extend the arm, raise the thumb, and, using the length of the thumb, to gauge the height of an object which is at a known distance, or the distance of an object of known height.

At 50 yards, a 6-foot-tall man is about the height of the upper part of your thumb; at 100 yards, a six-foot-tall man will be about the height of the thumbnail.

Artists, archers, soldiers and sailors have been holding their thumbs up for years, and not just for affirmation.

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