A small preview standing for a larger thing. Also the digit that opposes, which is why it means a grip.
Old English þūma (thumb), from Proto-Germanic *thūmon-, from a Proto-Indo-European root meaning to swell; the digit was named for being the thick one, and its opposing grip is why a small preview borrowed its name.
A small preview standing for a larger thing. Also the digit that opposes all the others, which is why the same word means a grip.