Green’s Dictionary of Slang

narc n.

also nark
[abbr. SE narcotics]
(orig. US)

1. narcotics; also attrib.

[US](con. 1960s) Courtwright & Des Jarlais Addicts Who Survived 223: At that time they had a ‘narc-vag’ law in Washington. If two or more addicts got together, they could arrest you.
[US]N. McCall Makes Me Wanna Holler (1995) 103: Some MPs with narc dogs walked up.

2. a narcotics agent.

[US]N.Y. Times 17 Oct. 44: Most speed freaks get to a point where they’re seeing narks in the trees with cameras.
[US]Time 26 Sept. 41: The culture has its own in-group argot: ‘bummers’ (bad trips) and ‘straights’ (everyone else), ‘heat’ (the police) and ‘narks’ (narcotics agents) and being ‘spaced out’ (in a drug daze).
[US]E. Tidyman Shaft 57: The police didn’t frighten him. The Narcs didn’t frighten him.
[US]E. Bunker No Beast So Fierce 31: He got on the stand and cut Joe loose, but the fuckin’ narks didn’t go for it.
[US]P. Corris ‘Stockyards at Jerilderie’ in Heroin Annie [e-book] Anyone at home in 88? [...] Junkies. [...] You a narc?
[US]C. Hiaasen Tourist Season (1987) 48: I’m beginning to think you’re a narc!
[US]Pileggi & Scorsese Goodfellas [film script] 111: All day I thought the guys in the helicopter were just local cops [...] but they turned out to be Narks.
[UK]Guardian G2 14 July 17: The narcs have been busy busy busy, what with the [...] busts of onetime teen pin-up Leif Garrett and Puerto Rican merengue star Alfred Cotto.
[US]F.X. Toole Pound for Pound 305: He checked the cathedral again for narcs.
[US]T. Dorsey Riptide Ultra-Glide 139: Just in case there’s an undercover narc on the beach.

3. any informer [note nark n.1 (1)].

[US]H. Feldman et al. Angel Dust 146: He asked me if I wanted to be a narc or an informer or all that bullshit.
[UK]Guardian Guide 25 Sept.–1 Oct. 52: He thinks he’s got the narc (‘The rat – it’s fucken Jimmy’).
[US]C. Carr Our Town 258: I thought it likely that people were taking pot-shots at a ‘narc’.

4. in ext. use, any unpleasant person.

[US]H.S. Thompson letter 5 Jan. in Proud Highway (1997) 603: Promoters, narcs, con men – all selling the New Scene to Time magazine.