Green’s Dictionary of Slang

1. (US Und.) an informer.

1960

C. Cooper Jr Scene (1996) 13: Flip [...] You sonofabitch!

2. (US black) a passive male homosexual.

1976		D. Ireland Glass Canoe (1982) 91: ‘Dirty bastard.’ ‘He’s a flip.’.
1992		Bentley & Corbett Prison Sl. 49: Flip A passive homosexual.

3. (US und.) the act of turning a criminal into an informer.

2012

L. Berney Whiplash River [ebook] [T]he DEA guys flexed their giant biceps and acknowledged [...] that they’d never saeen such a skilfull flip.