fink

Unknown; first attested in 1894. A connection to Yiddish as some propose is unlikely. Suggested origins include: * German Fink (“finch; frivolous or dissolute person; informer”) as finches are notoriously chatty birds in groups. If so, then Doublet of finch. Compare canary (“informer”). * Partly from the German theory, a fanciful association by students with the freedom of wild birds as opposed to caged ones. * The slang name pink for Pinkerton agents and their use as strikebreakers in the 1892 Homestead strike. If the term is from the corporate name, then it is of Scots origin, Pinkerton being from a place near Dunbar, which is from an unrecognized first element (possibly ultimately pre-Celtic substrate) and Old English tūn (“enclosure, homestead, etc.”).

来源：wiktionary