The line comes from Kill la Kill, the 2013 action anime series by Studio Trigger, where Nonon Jakuzure serves as one of the Elite Four student council members. Her flamboyant, music themed personality made many of her short exclamations popular as isolated clips online. The phrase nani sore is everyday Japanese for what is that, but her exaggerated delivery turned it into a recognizable anime reaction sound. It spread through soundboards, Discord servers, and short video edits, where fans and non fans alike use it as a quick expression of baffled disbelief.