Similar to the "Tea Bag", Clam Slap is the dipping/dragging of a woman's vagina over an enemy's face once the enemy has died. A complete and hillarious way of a female showing a person that their her bitch.
*WoW, Halo, CoD, etc character runs and rubs vagina over dead body's face*
*Player 1* Dude... you were just Clam Slapped.
*Player 2* Yeah... I smell like fish now... -_-