The joke works by taking the Navy Seal Copypasta, an aggressive monologue about being a highly trained killer, and dropping Japanese vocabulary into it wherever the original English sat. Words like watashi (I), anata (you), iimasu (say), chiisai (small) and desuka replace their English counterparts, producing a rant that sounds like an anime fan trying to intimidate someone while still practicing their Duolingo lesson1. The mismatch between the copypasta's macho posturing and the halting weeaboo grammar is the entire joke.

Over time, the opening three words got cut loose from the rest of the text and turned into a standalone slang expression. Posting 'nani the fuck' is basically saying 'WTF' with an ironic anime accent, and it works as a caption on confusing screenshots, weird tweets or absurd situations3. Urban Dictionary defines it as a description of 'an absurd situation in which everything must be taken seriously to the point of incredibly non-threatening incomprehension'3.

The full copypasta is typically pasted verbatim as a reply to someone acting tough online, mirroring how the original Navy Seal Copypasta is deployed. The shorter slang form, 'nani the fuck', is often used as a caption or comment on something confusing or surreal, especially content with an anime or Japanese pop culture angle3. Common convention is to keep the Japanese words untranslated so the mismatch does the comedic work.