Green’s Dictionary of Slang

(US) the FBI; thus an agent of the FBI.

1942		letter 27 Jan. in P. Jay Sel. Correspondence of Burke and Cowley (1990) 248: A publisher told me that a faithful phoebe had been going the rounds, presumably begging to be told that you were a C.P. because you didn’t support Franco.
1968		Atlantic Monthly Jan. 36: On their left stands a man in a very dark suit, with very dark tie, very dark glasses, very white shirt, and very bald head: a cop, Feebie, CIA, something like that.
1984		W. Diehl Hooligans (2003) 18: Local cops [...] get the runaround from the Feebies.
1986		C. Hiaasen Tourist Season (1987) 238: The clever Feebs used opaque envelopes.
1988		D. DeLillo Libra 45: Government cops, Feebees – the FBI.
1991		C. Hiaasen Native Tongue 177: ‘I pulled over a blue Ford sedan [...] Turns out to be a Feeb.’ ‘FBI?’ Skink perked up. ‘All the way down here?’.
2003		N. Green Angel of Montague Street (2004) 160: He had some guy from the feebies calling him. [Ibid.] 193: That’s why the feebs don’t follow him.
2013		T. Piccirilli Last Whisper in the Dark 234: ‘It was only a matter of time before the dead driver led the feebs back to accomplices’.