the act of sneaking into a bathroom, quietly lifting the lid off of the toilet tank, taking a deuce in the tank and then replacing the lid. this causes the toilet water in the bowl to be polluted after each flush. this is usually done at a party or too someone that you despise.
heath snuck into the frat house bathroom and left an upper decker in the tank, much to the dismay of the alpha betas.
by kirk May 27, 2005
A toilet prank, whereby someone defecates in the refill tank (water closet), causing each additional flush to deliver brown, poopy water into the bowl.
Jim was being such a jerk at his party, so I went upstairs to his bathroom and dropped an upper decker in his toilet.
by bodythunder November 12, 2008
When attending a persons home of who you dislike, take a shit in the toliet tank instead of the bowl. This is by far going out of the park and into the upperdeck where it will not be known until long after you depart their house.
When your wife makes you go to her boss's party who you would like to kill, go to the bathroom and leave him an Upper Decker and you'll know who the joke is really on.
by Noah Burnett April 28, 2006
To take a crap in the tank of someone's toilet and not flush afterwards. When the next person flushes your crap comes rushing into the bowl all broken apart. A lot will clog in the top. This is mean.
by Anonymous April 11, 2003
To take a crap in the top section of one's toilet so that when it is flushed, poo water flows back into the bowl.
by King Flump April 11, 2003
The act of removing the upper lid to a home toilet. leaving an imense amount of defacation inside. the next customer will go and then flush your shit into the bowl, repeatedly.
while in ohio, the clarion hotel wouldnt give us a room because we werent 21, so ryan anson gave them an upper decker
by friend July 31, 2003
When you take a crap in the tank of someones toilet instead of the bowl. Most often done to a sworn enemy or to your best friend.
by Helvis September 16, 2003