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.
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.
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
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"
The malicious act of making doodie in the water tank of a toilet, thus contaminating the toilet's clean water supply.
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.
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")