Definitions by NNDB

Jarn 

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

Jarn by NNDB March 22, 2012