Used on MMO forums the world (of warcraft) over, this meme phrase means that someone is a troll. Or trolling.
Phrase is said to originate, like so many other annoying memes, from lolcats
, in this case "longcat
Nubbins: ZOMG I waz dieded wai does game SUK DIK much plz plz nerf thing that kill ME!!!!11
Player: Obvious troll is obvious.