fink 
(fĭngk)Slang
(fĭngk)Slang n.
1. A contemptible person.
2. An informer.
3. A hired strikebreaker.
intr.v. finked, fink·ing, finks
1. To inform against another person.
2. To withhold promised support or participation: They said they'd help us, but then finked out.
[Perhaps from German student slang Fink, student not belonging to a student association (from German, finch; akin to Old English finc) or perhaps from such German compounds as Schmutzfink, dirty person, mudslinger : Schmutz, filth + Fink, finch (perhaps influenced by Rotwelsch (German underworld argot) Fink, Pink, man; akin to Frisian pink, little finger, penis).]