Big'n'Nasy describes the act of cumming into a womans, or mans mouth then proceeding to shit in their mouth also. After this, you make-out with the person with the cum/shit mixture and swap back and forth swallowing a little of the matter each time until it's entirely gone. Similar to snowballing
Guy1: Dude...I did a Big'n'Nasty with Victoria last night.
Guy2: Yea, I can tell...I can smell the shit on your breath.