Noun: First-person shooter video game slang.
Backrage refers to the act of hiding in the terrain behind one or preferably more enemies in a first-person perspective "shooter" online video game (or fortuitously stumbling upon same) and attacking them from the rear like a bastard and wiping them out by machine-gunning them in the back of the head. This is GeoffP's favourite thing to do in Return To Castle Wolfenstein and is another (of several) reasons he is hated there and banned from several sites, actually. Of course it has nothing to do with the swearing and the excessive cheating. It's not his fault the fuckers don't like wallhacks. Pfft.