The phrase is Italian and translates roughly to a crude promise to poop on someone, which is why it circulates as an intentionally immature joke rather than a serious insult. Its exact origin is unclear, so its life online is mostly through reposts, soundboards and short edits where the Italian delivery makes it funnier for non-speakers too. It sits in the same gross-out corner as a fart with reverb, used for shock, teasing and over-the-top reactions. Short spoken threats like this spread because they are instantly quotable and easy to drop into unrelated scenes.