The shout comes from SpongeBob SquarePants, where a recurring background fish, often called the My leg fish, suffers a leg injury in scene after scene and delivers the same panicked line each time. The repetition turned a throwaway cartoon gag into a running joke for viewers, and short clips of the moment spread through meme compilations, reaction edits and soundboards. It endures because the delivery is instantly recognisable even without context, so creators can use it as shorthand for exaggerated, cartoonish pain in almost any fail.