My Italian-American grandparents would often use the phrase "va fa Napoli" (or sometimes shortened to "fa Napoli"). You can hear the expression used on The Sopranos and in the Academy Award winning movie Green Book. My grandparents were Napolitano and learned the Napoli dialect from their parents), but they grew up and lived in the Bronx. From what I understand "va fa Napoli" is more Italian-American slang, maybe more specific to New York Italians, from the boroughs, with roots in Naples dialect (which is why Joey on Friends would use it, or Tony Soprano, or Tony the Lip in Green Book). My Italian teacher, who was from Naples understood it, but said most
Italian people don't use the expression. But my grandmother told me it meant "go to Naples" which was like saying "go to hell" because it's so hot down in Naples. I have no idea if this is true. But whatever the etymology, it's a negative expression but not vulgar, more akin to saying "get lost" than anything else... (certainly NOT "do it in the ass" as previously suggested.) There are so many Italian-AMERICAN expressions I picked up from my grandparents, most kinda vulgar, because they mostly spoke Italian in front of us to curse or insult someone. I've shared them with a Naples-native friend who understands these expressions perfectly, because they are still used, but often my pronunciation is slightly off--a product of the American influence and being diluted by generations who never learned to spell the words but could approximate the sounds of these expressions.