narc

someone who tells authorities about rule-breaking activities, a snitch

Tom: Jennifer is such a narc
Jack: yeah, she snitched on me for cheating on that test

Narc

In the game of legal matters a snitch is one who walks the walk and talks the talk. Talks the talk being the police and walks the walk being the felons. A snitch, a narc, a teller, a rat, an informant, a spy, a tattle teller.

Pipe down, I smell a narc.

Narc

someone who is snitching or is a snitch or a tattletale

"your not gonna narc on me are you?"

"your a narc"