Upperdecker

[Sonia]

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.

[Tina]

An upperdecker is when you poop in the toilet tank of someones bathroom, its the gift that keeps on giving

[Dave Paprocki]

A vengeful act of defacating in the tank of a toilet so as to odorize the bathroom with every flush for weeks until found. This is usually done at parties where you dont like the host/hostess.