(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.

Noun: a friend who is usually only called upon by a friend when a more primary/prominent friend is unavailable.

Taken from the term “second string” in an athletic competition situation. In football, if the star quarterback gets injured during a play, the second string is called off the bench to replace him/her. A “Second String Friend” is essentially benched until needed, if ever.

Synonyms: Plan B, Second Choice, Secondary Friend, Benched Until Needed

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.

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

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'

A term used to describe law enforcement of the narcotic's division. The word is a shortened form of narcotics and can also be spelled narcs. It can also be a term to call any police officer and does not limit itself to being a policeman of narcotics.

an undercover cop