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".
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!!
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.