The "Schlin" aka a "Gaper", is an asshole that exceeds in circumference (About the size of a quarter/half dollar)- Due to many acts of anal sex, especially double penetration. A schlin will cause pain to your butt when moved around.
Mike: Dude, Austin, remember that chick from the bar last night? We fucked and now she has a schlin!
Austin: That's gross, dude.