Alright kids, a jack nasty is where a boy usually high school or younger who can't get laid masturbates 'jacks off' into some tissues. Since he is often not one to properly discpose of them, they drift about the room and eventually make their residence beneath his bed.
The resulting hard, crusty tissue wad peeking out from beneath the mattress... Is a jack nasty.
Person A: What's with the tissues, dude?
Person B: Nothing!
Person A: Man you was makin' jack nasties!
Man that chick was so fine but I'll never get with her. I cry alone at night and make jack nasties.