The phrase comes from the traditional Spanish warning about el cuco, a bogeyman used for generations to caution misbehaving children. Online, the full line se la va a llevar el cuco has been clipped into a short reaction sound, shared in Spanish speaking chats and video edits as a mock serious threat. Its appeal is the contrast between an old childhood scare and a modern, clearly joking delivery, so it keeps circulating as shorthand for you are in trouble now without any real menace behind it.