As per the John Waters' film A Dirty Shame, an upper decker is the act of defecating in the tank of the toliet.
(As per my intoxication, an upper decker is my ability to write this while in the act (I'm currently the upper deck).)
"I started doing upper deckers, shitting in the top tank of the toliet, where no one would ever look to find the odor . . . "