Also referred to as 'Going Up Top,' the act of taking a shit in the tank of a toilet. Common courtesy not to flush. Used TP should be placed inside drawers, magazines, or the tank itself. Feel free to throw in any other items you can find in the bathroom during the process.

The upper decker is when you shit in the tank of the toilet.

The act of defecating into the upper tank of a domestic toilet.

The act of shitting into the tank (or upper portion) of a toilet. Done as a prank, it results in at least four foul flushes (also known as a "3-F qualifier") afterwards.

the act of taking a shit in the cistern of a pub or restaurent toilet to highlight dissatisfaction at service or food standards.

(Film Critique). When a movie starts out engaging and has great potential, but then the ending falls apart and the whole thing turns to shit. It's like finding an Upper Decker when you flush your toilet at the end of a party. It's there, and it's shit, you just don't know it yet.

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.