It's pretty much another way of calling someone a "finocchio".
Ok, I won't do that to you. Finocchio means homosexual(well, more derogatory, but this is a respectable forum). Busone, I think, is a slightly harsher way of puttin it...used more in the south I believe. But heck, what do I know. I'm korean. Just read your second post...literally, it means large "poop hole".