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 is long."
Nubbins: ZOMG I waz dieded wai does game SUK DIK much plz plz nerf thing that kill ME!!!!11

Player: Obvious troll is obvious.
by waru4u February 2, 2009