O.E.
þuma, from W.Gmc.
*thuman- (cf. O.Fris.
thuma, O.S., O.H.G.
thumo, Ger.
Daumen, Du.
duim "thumb," O.N.
þumall "thumb of a glove"), lit. "the stout or thick (finger)," from PIE
*tum- "swell" (cf. L.
tumere "to swell,"
tumidus "swollen;" Avestan
tuma "fat;" see
thigh). For spelling with
-b (attested from c.1290), see
limb. The verb meaning "to go through" (especially of printed material) is first found 1930, though the related sense of "soil or wear by handling" dates from 1644. Verb meaning "to hitchhike" is 1939, originally the thumb pointed in the direction one wished to travel.
Thumbnail sketch (1852) so called for its smallness. To be
under (someone's) thumb "be totally controlled by that person" is recorded from 1586.
Thumbs up (1887) and
thumbs down (1906) were said to be from expressions of approval or the opposite in ancient amphitheaters, especially gladiator shows, where the gesture decided whether a defeated combatant was spared or slain. But the Roman gesture was merely one of hiding the thumb in the hand or extending it. Perhaps the modern gesture is from the usual coachmen's way of greeting while the hands are occupied with the reins.
[thuhm] 
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