Basically, it is a game played by two or more males in which a muffin is placed on the ground, the players form a circle around the muffin, and the players must race to jizz on the muffin. The last person to jizz on the muffin must eat it. Origin unknown. Try this game with your camping buddies.
We were playing the Muffin Game and Brad, reluctantly, ended up eating the muffin.
In a fps, after getting a kill, the player stands on top of the dead body and proceeds to crouch up and down repeatedly, therefore it looks like he is "humping" his opponent. For me, this is best done on Halo 2 (Xbox live) because the person you kill can see that you are "humping" him.