The “nani” reaction became a common meme format through anime fandom and short-form video edits, where Japanese voice clips are used to heighten surprise. Rather than pointing to one definitive source, most users treat it as a stock anime reaction for disbelief, shock, or confusion. Its spread came from meme compilations, reaction videos, and remix culture on social platforms, where quick recognizable sounds outperform longer quotes. It is still popular because one word instantly communicates an over-the-top “what just happened” feeling.