You can fracture your thumb by smashing it too hard against an object or you can out of the blue just fall and get unlucky. Alot of times video game buttons can fracture your thumb, too.
The word thumb comes, via Old English, from the Proto-Germanic word tûmon meaning "thick" or "stout". So a thumb was called "tumon" because it was the thickest, stoutest finger. This, over the...