The act of defecating in the upper tank of the toilet. When the next poor unsuspecting person flushes the toilet they get a bowl of beef stew. the upper decker is a weapon of terror and should only be used on people who deserve it.
> My friends x-girl friend had a party and she left whith some other dude who looked like the fonz "Heyyy!". So I took it upon myself to leave her an upper decker