What is Xlap?
It must be written with a capital "x" - "Xlap"
A "Xlap" is a particularly aggressive attack, usually upon the face of the person in question.
A "Xlap" can be simplified as a backhand to the face - but it is very important to note that when threatening to "Xlap" someone, it is essential that you lift the right arm, so it is in line with the left shoulder (reverse if left handed) - therefore aligning the arm diagonally accross the body. From here, the Xlap backhand may be performed when required.
Poon: If that dude doesn't shut up, I'm gonna Xlap his ass!
Me: What on earth did you just do to that Chav?
Poon: I Xlapped his damn foice.