What does お前はもう死んでいる Omae Wa Mou Shindeiru mean?

Omae wa mou shindeiru means you are already dead. It is Kenshiro's declaration in Fist of the North Star, spoken to opponents whose pressure points he has struck and whose bodies simply have not caught up with the fact.

The sentence, taken apart

お前 (omae) is you, in a register that ranges from intimate to insulting depending on who is speaking to whom; from a stranger it is a challenge in itself. もう (mou) is already. 死んでいる (shindeiru) is the verb to die in its continuing-state form: not dies, not will die, but is in the state of having died.

That grammar is the joke's engine. Japanese has a form that treats death as a current condition rather than an event, so the sentence reads as a status report delivered to someone still standing.

Where it comes from

Fist of the North Star began in 1983, and its hero fights with a martial art that strikes pressure points, killing on a delay. The line is the formula that closes his fights: the work is done, the victim just does not know. The stock response, a bewildered nani, is the other half of the exchange, and that half has its own page here.

The meme

The exchange was fandom furniture for decades before it went general. Around 2017 the two lines escaped into remix culture, spliced onto everything from sports clips to birthday cards, and for most people who know the phrase now, the meme is the only context it has ever had. The register travelled with the delivery: nobody who learned the sentence from the meme can hear it said quietly.

What it demonstrates about the language

More than most memes, this one is a grammar lesson wearing a costume. English has to paraphrase the tense: already dead is an adjective doing its best, while the Japanese is a verb form built for exactly this meaning. The line is quotable in Japanese for the same reason it is slightly awkward in English, and the awkwardness is why the translation always keeps the original word order and rhythm in people's mouths.