A computer-roleplaying-game player, whose character is such a high level that the playing is actually getting a bit boring. It can either mean a player who moves very slowly in the game, preferring to level up than follow the plot; a player who is stuck in the plot and wanders aimlessly trying to find a way to continue; or a player who has almost completed the plot, but doesn't want to game to end and is just wandering, unwilling to finish.
The term originates from the Douglas Adams's novel "Life, the Universe, and Everything", where an immortal grey-green alien by the name of Wowbagger the Infinitely Prolonged travels the universe in order to insult everybody in alphabetical order. Also, WoW stands for World of Warcraft, a large, popular multi-player online roleplaying game.
"Oh I don't play that game anymore, I became such a WoWbagger in it."