Back acne; acne that is on one's backside
Joe: Dude, have you seen Jared in the locker room?
Jack: I know, he must have a bad case of backne
(n) acne that forms on the back area, this can include, shoulders to lower back...not to be confused with assacne, that forms on the buttocks, particularly unpleasant to see on white pasty arses.
She shouldn't have worn that backless dress, her backne is really raw and angry.