Named for Damage, the main character from Malice, which was a commercially produced total conversion for the original Quake, released many many years ago.
Malice, apart from being extremely cool, was unique in that it had cutscenes (pretty much unheard of in FPSs back then), weapons that had to be reloaded, and cool stuff like mini-subs and parachutes, also unheard of. Damage himself was a tough, hard-talking mercenary-for-hire and the game's main protagonist. His first action in the very first cutscene was to shoot the original Quake guy (who was about half Damage's height) in the back for non-payment of a debt.
Loved the guy immediately and adopted his name from that day forth. Been using it ever since.
"Hey Damage, you wouldn't shoot a guy in the back, would ya?"
"Guess again, asswipe."
Damage is my avatar pic, too. Oh, the ZP is simply to help me distinguish from the numerous other Damage usernames I've got on other sites. Named for Yahtzee, of course.
http://en.wikipedia.org/wiki/Malice_(video_game_mod)
And now you know.