Thumb
Old English thūma, of West Germanic origin; related to Dutch duim and German Daumen, from an Indo-European root shared by Latin tumere ‘to swell’. The verb dates from the late 16th century, first in the sense ‘play (a musical instrument) with the thumbs’.
wiktionary
From Middle English thombe, thoume, thoumbe, from Old English þūma, from Proto-Germanic *þūmô (compare West Frisian tomme, Dutch duim, Low German Duum, German Daumen, Danish tomme, Swedish tumme), from Proto-Indo-European *tūm-(“to grow”) (compare Welsh tyfu(“to grow”), Latin tumēre(“to swell”), Lithuanian tumėti(“to thicken, clot”), Ancient Greek τύμβος(túmbos, “burial mound”), Avestan 𐬀𐬨𐬏𐬙 (amūt, “strong”), Sanskrit तुम्र(túmra, “strong, thick”)). The parasitic ‐b has existed since the late 13th century. [1]
etymonline
thumb (n.)
Old English þuma, from Proto-Germanic *thūman- (source also of 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- "to swell." Unetymological spelling with -b (attested from late 13c.) is perhaps by influence of dumb; also compare limb (n.1).
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."
Phrase rule of thumb attested by 1680s (the thumb as a rough measure of an inch is attested from c. 1500). 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.)
"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.
