Real Dictionary

thumb
countable noun: The thumb of a glove is the part which a person's thumb fits into.
countable noun: Your thumb is the short thick part on the side of your hand next to your four fingers.
verb: If you thumb a lift or thumb a ride, you stand by the side of the road holding out your thumb until a driver stops and gives you a lift.

thumb in American English
the short, thick digit of the human hand that is nearest the wrist and is opposable to the other fingers
to handle, turn, soil, or wear with or as with the thumb
noun: the short, thick, inner digit of the human hand, next to the forefinger

thumb in British English
noun: the first and usually shortest and thickest of the digits of the hand, composed of two short bones