Upperdecking 

Removing the lid of the back of someone's toilet and shitting in the clean water. Results in shit running into the boil with every flush

That bitch pissed me off, so I upperdecked her shitter

Upperdecking by Dan April 5, 2004

Upperdecking 

This one takes practice. This maneuver requires a toilet with a tank above it, like the ones in most homes. Instead of crapping in the bowl, you shit in the tank. Now don't flush. When the following victim flushes, the rancid waste fills the bowl. If you play your cards right, it may ferment. Great prank for those lame partys.

"I gave that toilet a nice upperdecking, Just wait till she comes outta there"

Upperdecking by CharlesVA August 5, 2006

Upperdecking 

Using the restroom, going number one or two in the water tank of a fellow man's toilet. when he flushes the pee or shit flows out.

The other night we were upperdecking, it was funny as shit.

Upperdecking by douchebagness May 16, 2007

underdecking 

conspiring against an authority figure to take synchronized shits underneath said authority figure's deck. Usually performed in groups of two to five people. In extreme cases, shit burritos are fashioned with paper towels and left to be found.

Oh, no. Have you seen the kids? I hope their not underdecking us again!

underdecking by underdecked July 2, 2009

upper decking 

The process of taking a shit in the tank of a toilet instead of the bowl so when the next person flushes, all the shit goes into the bowl

Taking a shit in the tank instead of the toliet is upper decking.

upper decking by Jay December 26, 2003

Upper Decking 

giving John Barrowman a blowjob on a transatlantic flight

"In the middle of the night ... on a 747 on the upper deck" (hence the term "upper decking")

Upper Decking by qwapos September 7, 2012

Upper Decking 

Deficating in the tank of someone's toilet, rather than the bowl. Often employed in malicious context.

Suzy could not figure out why her toiled would flush only brown water. Little did she know that Johnny had upper decked that bitch the previous night.

Upper Decking by Korthos July 27, 2004