Other forms: finks; finking; finked
A fink is someone who snitches or tattles, like your sister who told your parents that you were the one who ate the last chocolate chip cookie. That fink!
Fink is a North American slang term that originally meant "informant" or "strikebreaker" and expanded to be a general pejorative term for a jerk or an unpleasant person. Experts aren't sure about the word's origins, though many suspect it comes from German; Fink means "finch," but it's also used for "frivolous or dissolute person." In any event, you'll know a fink when you see one, because they'll be tattling on you.