Herfing the derf

It is discribed as "Fucking the fucked"; To have sexual intercourse with a fucked up person. May sound unusual in a way to make people just say: "Was that a threaten?" or Having no idea of what has been said..

DUDE!? I totally saw you and ashley herfing the derf with each other.

Parents: Did you do anything dirty in tha room?
Me: No? I was just Herfing The Derf
Parents: Oh Okay good?