Something from the anime/manga series ‘Fist of the North Star’, meaning ‘you are already dead.’ And then the person it has been said to dies. If you hear this said, it is completely unacceptable to not say ‘Nani?!?!?’.
This is only used in the highest profiles of the highest edgy meme lords. it can be used to absolutely destroy an opponent in battle. Only the best can use it. It means “you are already dead” but will instantly kill your opponent in an anime battle.
'You are already dead' in Japanese. Legend says that if you say "Omae wa mou shindeiru." to an opponent, their response will always be "NANI?" meaning 'WHAT' in Japanese. Then said opponent will die. If 'Nani' is not said, then...? The opponent will die anyway though.