thumbyoudaoicibaDictYouDict[thumb 词源字典]
thumb: [OE] The thumb is etymologically the ‘swollen’ part – an allusion to its greater thickness than the other fingers. Along with its relatives German daumen and Dutch duim, it goes back to a prehistoric West Germanic *thūmon. This in turn can be traced to Indo- European *tum- ‘swell’, which also produced English tumour and tumult. The b in thumb appeared in the early Middle English period, when it was still a two-syllable word (thumbe), and at first was pronounced, but it has fallen silent over the centuries.
=> thigh, thimble, tumour, tumult[thumb etymology, thumb origin, 英语词源]
thumb (n.)youdaoicibaDictYouDict
Old English þuma, from Proto-Germanic *thumon- (cognates: Old Frisian thuma, Old Saxon, Old High German thumo, German Daumen, Dutch duim "thumb," Old Norse þumall "thumb of a glove"), literally "the stout or thick (finger)," from PIE *tum- "swell," from root *teue- (2) "to swell" (see thigh). For spelling with -b (attested from late 13c.), see limb.
In some of the IE languages there is a single word for "thumb," which is called the "big finger," like NE big toe. Many of the single words are of similar semantic origin, based on the notion of "stout, thick." [Buck]
Compare Greek megas daktylos "thumb," but Greek also had antikheir, literally "what is opposite the fingers." Italian pollice, French pouce are from Latin pollex, perhaps formed (on analogy of index) from pollere "to be strong."

To be under (someone's) thumb "be totally controlled by that person" is recorded from 1580s. 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.
thumb (v.)youdaoicibaDictYouDict
"to go through" (especially of printed material), 1930, from thumb (n.), though the related sense of "soil or wear by handling" dates from 1640s. Earlier as a verb it meant "to play (a musical instrument) with the thumb" (1590s). Meaning "to hitchhike" is 1939; originally the thumb pointed in the direction one wished to travel. Related: Thumbed; thumbing. To thumb (one's) nose as an expression of derision is recorded from 1903.