Narc. 21 Jump Street originally ran on the Fox network from 1987-1990. 21 Jump Street is the headquarters for a squad of police officers who specialize in investigations relating to young people.
Instead of saying : "That fucker Ronnie, he is such a narc. He told the cops I was smoking pot in the bathroom.", say : "That fucker Ronnie is so the guy from 21 Jump St."
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.