"Dude, it was Fission Mailed, they just tried to confuse you, now you gotta start all over again cause you didn't save. Pwned!
Origin: Metal Gear Solid 2 near the end of the game, there's a screen that says that very phrase that's supposed to mimic the "Mission Failed" message you get when you normally die in said game.
it means "Mission Failed". which is used as a derogative term to slander someone/ someones argument that they've put forward in a debate, it is both patronizing and shows that you're putting an end to the argument.
Considered to be part of l33t sp33k. Gaming slang. Often used as the end of a sarcastic comment.