If you are one of the blessed few to have escaped this particular aspect of multiplayer internet gaming, then allow me to give you a rancid spoonful from the bowl of "stuff you didn't want to know about":
In the realm of videogames, "teabagging" refers to the act of positioning your avatar over the face of a fallen enemy and repeatedly tapping the crouch button in order to demonstrate your comprehensive superiority over them. It is not considered to be the zenith of wit. It's roughly the equivalent of shouting "IN YOUR FACE, BE-YOCH!" when you jump someone in checkers.
I don't know that the practice was invented by Halo players, but that seems to be where the act gained traction and eventually widespread adoption.
So thank you, Halo. Thank you for your unique contribution to our proud culture.
Shamus Young is a programmer and writer by trade, videogame nitpicker by inclination. If you have the patience for more of his ramblings, they can be found at ShamusYoung.com.