The Act of taking a shit in the Upper Tank of the toilet as a prank.
I JUST LEFT THEM A UPPER DECKER!!!
by DaddyFatSizzles March 29, 2011

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When you have a massive deuce that must come out when your at a big party. You remove the top lid to the toilet, and release all the poo into it. When done, put the lid back on, and walk out like nothing happened.
I just put an upper decker in her toilet, HAHA!

There gonna be lookin for that upper decker for weeks,HAHA!

by boiiiiiiiiiiiiiiiiiiiiiiiiiiiie January 16, 2006

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A modification on the "classic" upper decker. In a public toilet setting (where there are no tanks/commodes), you improvise and shit in the urinal.
When Hank got fired from his job, he pulled an upper decker in the company bathroom.
by David C August 12, 2005

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Unleashing the biggest load of crap in the top portion of the toilet.
"I wasn't able to get a beer at this party, so I took it upon myself to give him an upper decker in the toilet".
by mark abestin December 18, 2003

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to leave a nice steaming terd on the toilet seat, enhancing the savory odor for the next user of the john; especially irritating to females, dudes not so much. Not to be confused with a Back Door Bandit which involves removing the lid of the reservoir tank and much better balance
eww, gross!!! some fuckwad left a upper decker in there!!!!! take me somewhere to tinkle, dammit!!
by coldbowl May 17, 2007

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To put toilet paper over someone elses poo when the toilet is clogged and then take a shit on top of the original poo.
I shit on eric's poo because he clogged the toilet and there was nowhere else to go.
by grind January 16, 2004

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