Alternate sex partner being seen as the same time as another.
I have one girlfriend and 3 side-fucks.
von Emi, Will, Hunter 7. Dezember 2006
The act of creating a new orifice in a person's side, and then raping the person using the new hole.
"Billy was mad at Timmy because Timmy had cut off Billy's left nut in a prank gone wrong, so Billy used his pocket knife to cut a hole in Timmy's abdomen, then sidefucked him. Timmy learned his lesson"
von cheesegrass 4. September 2007