Green’s Dictionary of Slang

an unpleasant person, with overtones of working as an informer; also attrib.

1959		San Bernardino Sun (CA) 8 May C12/2: The wounded assistant leaped behind a chair rind screamed, ‘You rat fink!’ at his boss.
1963		J. Blake letter 7 Sept. in Joint (1972) 222: Doug belts me around the room at length, calling me a ratfink.
1977		E. Torres Q&A 42: I don’t talk to that rat fink without a surgical mask.
1989		H. Rawson Dict. of Invective (1991) 147: A modern embellishment, from the 1960s, is ratfink, meaning an informer, traitor, or other person who is even more detestable than your average fink.
1996		K. Lette Mad Cows 143: It’s just your life philosophy, pseudo-male Feminism, rotten, ratfink, dirty-dingo lying and dress sense I can’t tolerate.
2001		‘Randy Everhard’ Tattoo of a Naked Lady 130: I wondered which lousy rat-fink fingered me.
2021		J. Ellroy Widespread Panic 176: I’m a snitch. I’m a rat fink [...] a doofus double agent.