The phrase is everyday colloquial German, a contracted form of was zitterst du so, and it circulates online mainly as a standalone reaction sound rather than a clip tied to a known film or show. On soundboards and in edited videos it functions as a quick verbal jab, borrowed by German-speaking creators and meme editors who need a short line to point out trembling, anxiety or dramatic fear. Its brevity and clear delivery keep it reusable across gaming, chat and fail contexts.