Green’s Dictionary of Slang

(US Und.) given to acting as an informer, untrustworthy.

1940		S. Longstreet Decade 275: Call us vested interests, sweat shops, yellow-dog contractors, fink employers [...] but we made a damn good pot.
c.1951		N. Algren ‘Watch Out for Daddy’ in Entrapment (2009) 130: Christian Kindred — finky liar — you good and well know that ain’t no New York Central.
1956		J. Blake letter 22 Dec. in Joint (1972) 101: This is such a fink town, I despair of finding a straight fence. The stuff is too hot, the town too small.
1959		W. Burroughs Naked Lunch (1968) 180: His dossier contains three pages of monikers indicating his proclivity for cooperating with the law [...] Ab the Fuzz Lover, Finky Marv, The Crooning Hebe.
1970		E. Thompson Garden of Sand (1981) 39: The philosophy of generations of Protestants was being scorned by executive orders, delivered by finky little bureaucrats from some alphabet office in Washington.
1972		M. Rodgers Freaky Friday 54: What’s so great about a person who pulls that kind of finky trick?
1973		N. Algren Last Carousel (1997) 358: ‘That finky cop,’ she said.