Green’s Dictionary of Slang

to betray someone to the police, spec. a drug dealer or user to the narcotics police.

1979		T. Alibrandi Custody 289: ‘You’ll have to call your probation officer and let him know you didn’t keep your appointment,’ I said [...] ‘Sure. Call the pig and narc on myself. You must have broken tools in your garage,’ she sneered.
1980		A. Pearl Dict. Popular Sl. 105: Narc...v. to inform [HDAS].
1997–2001		Online Sl. Dict. 🌐 narc v 1. to report a person or group to the authorities after associating with that person or group.
2001		‘Randy Everhard’ Tattoo of a Naked Lady 119: Don’t narc on me, and I won’t narc on you.