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.
ayon kay Korthos ika-27 ng Hulyo, 2004
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"
ayon kay CharlesVA ika-05 ng Agosto, 2006