When you have sexual relations with a girl who you later find out has had sex with one of your friends or someone who you once knew. Then you are slapping-dicks with them. Much similar to sloppy-seconds.
Bob: You know that girl Jenna?
Jimmy: Ya. Why?
Bob: I totally beat those cheeks last night.
Jimmy: Oh ya? Me too. A few weeks back.
Bob: Ah man! I was slapping-dicks with you.