Old English þuma, from West Germanic *thumon- (cf. 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" (cf. Latin tumere "to swell," tumidus "swollen;" Avestan tuma "fat;" see thigh). For spelling with -b (attested from late 13c.), see limb.
To be under (someone's) thumb "be totally controlled by that person" is recorded from 1580s. Thumbs up (1887) and