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1.
the act of fucking a girl who is way below any normal standards
person 1:so i heard you went derfing last night.
person 2:yeah, I took pcp.

I wanted to go derfing, to I found the girl nicknamed Trailer Trash.

Bonus points if they have a kid.
by fred v. September 14, 2006
 
2.
When a guy and girl are sexually intimate, the practice of the guy to alternate between bottom and top holes (anus and vagina). No particular pattern of action is required, and the switch-off can be completely random (ie: top, top, bottom, top, bottom, top, bottom, bottom, bottom, top, fist).

Create special patterns to signify festive or celebratory events and occassions; re-live these occassions through fantastic, nostalgic sexual practices!
Donnie: Tim was derfing Julie and covered her vagina with shit.
Darcy: Goddam! O_O
by Pwnforce United December 23, 2005