An acronym for jackass, retarded nukka (j.a.r.n.). It's meant to slander a person's character after he/she makes a rude comment or performs an act associated with a jerk.
A bad story (see sharn) told by somebody whose name begins with the letter 'J'. When a jarn is begun it must be stopped as soon as possible. It is both painful for those who hear it and embarrassing for the teller of the jarn.