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1.
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.
"What a jerk move. Brian is such a jarn."

"I can't believe you brought up the time everyone got crabs, you jarn!"
by Glenda_ October 06, 2006
 
2.
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.
Sound the jarn-alarm, Jonny wants to say something
by NNDB February 10, 2012