1. It's a sentence from Rome
slang. It means a self-important persone, but in fact, a
guy no one knows coming from a
little town no one knows.
2. It could be used also to say "you know very well who"
2. A
husband comes back home and shuts
the door. The wife asks: "Who is?" - the
husband: "Fra Cazzo da Velletri" (= who else do you wonder it could be?)