Used commonly by MMORPG players to convey that they are unimpressed or annoyed to the point that they actually type it out.
Usually the player is trying to show that they clearly know more about a video game and are mocking other peoples knowledge of said game.
Unfortunately it's just a fucking game and knowing more about interactive fiction than having social/survival skills classifies the player as a total douchebag and most likely to be the first to die when presented with a life or death situation.
noob player: "So how do I kill stuff?"
douchebag player: "*sigh* Stop playing the game, your stupid question annoys me."
noob player: "what a douche"
Real World Scenerio
Frantic Person running by: "Oh my god! Some guy has snapped and is on a killing spree! Get the fuck out of here!"
douchebag player, thinking to him/herself: "*sigh*, if only I could use invisibility, just like in my favorite game, I could....Ahhh!"
The douchebag player has been killed by the rampaging psychopath.