Like "more cowbell", but twice as good, due to the 'moar
(More cowbell has often been used in reference to the situation with Will Ferrel, but in all actually, it's just another way to say "awesome".)
You have just shot some poor n00b
's head off in a game that you know you probably play far too much, and someone on your team says "moar cowbell!", likely in all caps, with multiple explanation points, to declare your nerdy awesomeness.