Nark

[Tina]

(nark can also be spelled narc or narq)
1: To be a nark is to be someone that will report any misbehaving to a higher authority like the police, parents and teachers.
2: To nark, or to nark someone out is the action of reporting the misbehavior; also called narking.
3: To be narked is, obviously, the past tense of narking.

[Donald Trump]

A long time ago Nark (also spelt narc or narq) referred to narcotics officers working undercover. When the undercover officer reported his findings to whoever he was working for, he was narking on his druggy friends.
Nowadays, a nark is basically a tattle-tale.

[Tina]

informer, someone who has got their nose into your business. Romany word naak means nose. Not to be confused with the later American word with similar meaning which is an abbreviation for Narcotics Agent

[Tina]

the word is NARC as in one of those people who look really young that the cops put in schools as a fake student who busts people for NARCotics
slang in terms of someone who told on you

[Sonia]

noun-- Australian term -- one who complains and spoils other peoples enjoyment
verb-- British term -- to annoy someone or spoil their enjoyment

[Britney]

Someone who rats out fellow friends to deans, or other authorities.

[Rihanna]

an undercover cop