Merriam-Webster's Collegiate® Dictionary, 11th Edition
THUMB
I. noun
Etymology: Middle English thoume, thoumbe, from Old English thūma; akin to Old High German thūmo thumb, Latin tumēre to swell Date: before 12th century the short thick digit of the human hand that is analogous in position to the big toe and differs from the other fingers in having only two phalanges, allowing greater freedom of movement, and being opposable to each of them, the part of a glove or mitten that covers the thumb, a convex molding ; ovolo ,
II. verb
Date: circa 1647
transitive verb
1. to leaf through (pages) with the thumb ; turn , to soil or wear by or as if by repeated thumbing , to request or obtain (a ride) in a passing automobile by signaling with the thumb, intransitive verb to turn over pages , to travel by thumbing rides ; hitchhike