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1.
The opposite of the upper decker, which involves leaving a fresh pile of feces on the floor next to the toilet, but none actually in the toilet. The lower decker can be used in many situations, such as the workplace or enemies homes.
"Dude, I totally did a lower decker in the men's bathroom."
by simechkr May 01, 2006
 
2.
The act of defecating in a urinal. Commonly done in a old fashioned urinal system in which the water does not flush, leaving a wretched stench and a very unusual surprise for any unsuspecting person who wishes to urinate.
"This school dance sucks!, i'm going to take a lower decker in the bathroom."
by Papa Panda December 22, 2011