A Dirty Mott, AKA Upper Decker, is the disturbing act of defacating in the upper tank region of a toilet. While something of this nature requires superb balance, bowel control and ass-eye coordination, perhaps the most important skill is simply the balls-out willingness to shit in someones toilet tank.
"Good god, somebody hit our toilet with a dirty mott!"
"What the fu..." Blake trailed off in disgust as he realized the brown substance flushing INTO the toilet bowl from the tank was the result of a dirty mott.