The line comes from Fist of the North Star, the long-running Japanese manga and anime series about the martial artist Kenshiro, who defeats enemies with pressure-point strikes that destroy them moments later. In the story he often warns a foe that they are already dead before they realise it, and the foe answers with a stunned Nani. Clips of the exchange circulated widely online as anime memes and reaction videos, helped by its short length and clear setup and payoff. It remains popular because the joke works even for viewers who have never seen the series.