Upper Decker is the classic prank you play on the host, once he or she declares you unwelcome in a house party or other event at their place.
It is basically the act of squatting over the tank of the toilet and dumping in it, so that the next flush will present someone with your Kinder egg.
It is no surprise that the person, who came up with the idea kept their name to themselves, since not everyone would boast with the fact that they took a dump in the tank of a toilet, to enrage the owner of the bathroom piece.
It most likely had been present in the repertoire of humorous pranksters since the last century, or possibly even further in the history of the toilet; as we were taught by South Park, the flush toilet had been part of European lifestyle since the late 16th century, thanks to Sir John Harington.
This provided plenty of time for humanity to come up with the idea of laying a cable in the upper part of the toilet.
Urban Dictionary defined this hygienic and hilarious practice as early as 2003, with several other entries written on the subject, one even going as far as giving instructions on how to do it.
The openness of the internet had even allowed victims of the prank to share their misery, as their toilet is spewing dump in the bowl, instead of cleaning it with water.
It also led to people become more familiar with the practice and do it in the bathrooms of their enemies.