spuckie is a word used to describe a submarine (sub) sandwich. It comes from the Italian word "spucadella" which is an italian sandwich roll. It most likely came from the Italians in the North End and elsewhere... (see full definition)
As for the word's origin in Southie (South Boston), most sources seem to leave that out.
As for the word originating from the Italian "spacato", I must plead ignorance. I don't speak Italian, and I am not familiar enough with Boston English to give a firm ruling on this.
Can anyone help out with this?