1 definition by Grant Herman
The upperdecker is a prank that you can pull on a friend or loved one, or more likely someone who deserves it. An upperdecker is defined as defecating in the upper tank of a toilet, allowing it to fester for as long as possible and sending a chilling brown residue into the bowl upon each flush.
At a party where they run out of beer one might like to leave an "upperdecker" to show their disapproval.
by Grant Herman August 31, 2004