To poop on the floor on the top level of a double decker bus.
Someone was in the first level bathroom so Becky pulled a British Upper Decker.
by @catfur June 25, 2012
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When one is frustrated and angered to the point of giving the accused a most-deserved upper decker and feels the need to voice their anger- they must, just before dropping a lung in the toilet tank, shout loudly- "This is Sparta!!!" as spoken in the movie 300. After this is spoken, one would shit with vengeance into the tank.
Angry person- That bitch of a waitress ignored our table for over half-an-hour because they add automatic gratuity to the check, so I will leave them my opinion of their service in their toilet tank in the form of a Spartan Upper Decker.

Friend 5 minutes later- Did you do it?

Angry person- I showed no mercy!
by Litaker April 7, 2011
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As with a traditional Upper Decker (where the attacker poops into the reservoir of the victim's toilet), but with the addition of gelatin powder, which, coupled with the cold water temperature and cool porcelain construction of the reservoir, causes allows the mixture to set into a poop jell-o which requires the victim to manually scoop it out
"Hey John, did you hear? someone left Tim an Atomic Upper Decker during the party on friday!"
"Wow, he must have really pissed someone off."
by Judge)Mental December 19, 2017
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The act of filling the top reservoir of someones toilet with liquid feces. Preferably, after the mass consumption of spicy foods such as vindaloo or chili.
Oh my god, an Upper Decker Demolator! I keep flushing and it keeps coming back!! Ok, you keep flushing, I'm going for more bleach. And some mints, in case you have to siphon it out...
by dissy614 January 24, 2009
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The art of sneaking into a frat party for the purpose of taking the most massive grueling shit in the upper tank of the frat house's toilet. That way when they wake up to clean the place in the morning, they are greeted with the most fowl awesomeness that only frat boys deserve.
Dude, I totally dropped a Greek Upper Decker in ZBT this past thursday.

Sweet. Those guys are dicks. You deserve a trophy man!
by The Greek Upper Decker January 10, 2009
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The act of sharting in public and then taking your shit pasted underwear off and disposing of it in the tank of the toilet
Man 1. Boo boo has been gone for awhile

Man 2. He's probably taking a shit

Man 1. He farted before he left so likely

Person in question....we need to leave

Later admits he put his shitty underpants in the tank of the toilet of a public setting to avoid being embrassed

Now their shit covered under pants are worse then discovering actual shit in the tank

The upper decker delux
by Takindumps4200 June 29, 2022
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When taking a dump with such tremendous force that it actually pushes the crap up into the tank from the bowl.
Man that Thai food packs a punch - i just had a Reverse Upper Decker in the shithouse!
by Chief Brody April 8, 2010
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