fink (n.)

1902, of uncertain origin, possibly from German Fink "a frivolous or dissolute person," originally "a finch" (see finch); the German word also had a sense of "informer" (compare stool pigeon). The other theory traces it to Pinks, short for Pinkerton agents, the private police force hired to break up the 1892 Homestead strike. As a verb, 1925 in American English slang. Related: Finked; finking.