In a first person shooter (
FPS) game, a "pinger" is a kill which was only achieved because the killer's
ping was higher than the killee's.
For instance, if two snipers are at opposite ends of the map, both aiming at each other and the game begins, then basically whoever has the lowest ping will kill the other. This is a pinger.
Some games have lag compensation to make things better for people with low pings - this is very useful for those of us with 56k modems.