thumb

Concise Oxford English Dictionary © 2008 Oxford University Press:
thumb/θʌm/

▶noun the short, thick first digit of the hand, set lower and apart from the other four and opposable to them. ▶verb

• 1 press, touch, or indicate with one's thumb.

• 2 turn over (pages) with or as if with one's thumb.

  ■ (usu. as adj. thumbed) wear or soil (a book's pages) by repeated handling.

• 3 request or obtain (a free ride in a passing vehicle) by signalling with one's thumb.

– phrases
thumb one's nose at informal show disdain or contempt for. [with ref. to the gesture of putting one's thumb on one's nose and spreading the fingers.]
thumbs up (or down) informal an indication of satisfaction or approval (or of rejection or failure). [with ref. to the signal of approval or disapproval used by spectators at a Roman amphitheatre (although the Romans used the symbols in reverse).]
under someone's thumb completely under someone's influence or control.

– derivatives
thumbed adjective,
thumbless adjective.

– origin OE thūma, of W. Gmc origin.