“Nani sore” is a common Japanese expression meaning “what’s that?” Online, short clips of it are widely used as a reaction for confusion, disbelief or mild shock. Even when the exact source clip is unclear, the phrase spread through meme edits, anime-themed videos and short-form posts because it is quick, expressive and easy to understand from context. It fills a similar role to a shocked sound, especially in fast edits where creators want a puzzled reaction without needing a long voice line.