thumb / [unvoicedth]əm/ • n. the short, thick first digit of the human hand, set lower and apart from the other four and opposable to them. ∎  the corresponding digit of primates or other mammals. ∎  the part of a glove intended to cover the thumb.• v. [tr.] press, move, or touch (something) with one's thumb: as soon as she thumbed the button, the door slid open. ∎  turn over (pages) with or as if with one's thumb: I've thumbed my address book and found quite a range of smaller hotels | [intr.] he was thumbing through that magazine for the umpteenth time. ∎  (usu. be thumbed) wear or soil (a book's pages) by repeated handling: his dictionaries were thumbed and ink-stained. ∎  request or obtain (a free ride in a passing vehicle) by signaling with one's thumb: three cars passed me and I tried to thumb a ride | [intr.] he was thumbing his way across France. PHRASES: be all thumbs inf. be clumsy or awkward in one's actions: I'm all thumbs when it comes to making bows.thumb one's nose at inf. show disdain or contempt for.thumbs up (or down) inf. an indication of satisfaction or approval (or of rejection or failure): plans to build a house on the site have been given the thumbs down by the Department of the Environment. under someone's thumb completely under someone's influence or control.DERIVATIVES: thumbed adj.thumb·less adj.

thumb the breadth of the thumb, taken as equal to an inch, was formerly used as a measure; in the cloth trade, it was customary to allow a thumb in addition to each yard of cloth measured. The word is recorded from Old English (in form thūma), of West Germanic origin; it comes ultimately from an Indo-European root shared by Latin tumere ‘swell’.
thumbs up (or thumbs down) an indication of satisfaction or approval (or of rejection or failure), with reference to the signal of approval or disapproval, used by spectators at a Roman amphitheatre; the sense has been reversed, as the Romans used ‘thumbs down’ to signify that a beaten gladiator had performed well and should be spared, and ‘thumbs up’ to call for his death.

See also bite one's thumb at, all fingers and thumbs at finger.

thumb sb. OE. þūma = OS. thūma, MLG., MDu. dūme (Du. duim), OHG. dūmo (G. daumen) :- WGmc. *þūman-; repr. IE. *tum-, one of several extensions of *tu- swell.
Hence vb. XVI.

thumb (thum) n. the first digit on the radial side of the hand. It can oppose the other four fingers of the hand.