The sound comes from The Milky Waif, a classic Tom and Jerry cartoon in which Jerry looks after a young orphan mouse. In that short, the normally small and quick-witted mouse produces an unexpectedly loud roar, a classic cartoon gag built on size versus sound. Fans later lifted the clip for soundboards and meme edits, where its short length and instant contrast make it easy to reuse. It survives because the joke needs no explanation: a tiny mouse roaring sounds funny every time.