The act of shitting into the tank (or upper portion) of a toilet. Done as a prank, it results in at least four foul flushes (also known as a "3-F qualifier") afterwards.
"Man, I'm gonna fucking puke."

"Dude, how come?"

"Man, someone dropped an upper decker in the toilet, and the water splashed my ass and sack."

"Dude, that sucks."

"Man, I think I got a piece of corn stuck to my sack!"
by papamikedelta October 24, 2009
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A party trick
The trickster takes a dump in the cistern of the host's toilet, resulting in rusty water for months unless the offending turd is fished out and flushed away.
I went to that bastards party the other day, drank his beer, fucked his girlfriend and left him a top decker for the morning.
by Squealpiggy September 19, 2005
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As per the John Waters' film A Dirty Shame, an upper decker is the act of defecating in the tank of the toliet.

(As per my intoxication, an upper decker is my ability to write this while in the act (I'm currently the upper deck).)
"I started doing upper deckers, shitting in the top tank of the toliet, where no one would ever look to find the odor . . . "
by iloverobots01 January 9, 2006
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using the water retaining bowl of the toilet, instead of the toilet itself.
that beyotch pissed me off, so I ate 3 large burritos, took some Ex Lax, went over to her house and left her an upper-decker to deal with.
by Amanda Huggankiss August 2, 2006
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While at a respected friends house retire to their restroom, stealthily remove the lid from the back tank of their toilet and steady your cheeks directly above it. Begin defecating while trying to keep as quiet as possible. When finished, wipe all remaining fecal spatter from your anus and hide the used toilet paper in the cabinet under said friends bathroom sink. When the toilet is flushed by the next user the bowl will be rinsed with your feces. A firm stool will cause repeated minor fecal rinsing, while viscous, soupy stool will flush out all at once filling the toilet thus causing repeat flushing until clean water is present.
The upper decker was rejoiced by all who were present.
by A Tortured Soul March 5, 2006
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To receive a blowjob whilst depositing feces in the upper tank of a toilet.

A combination of a blumpkin and an upper decker.
As vengeance for not being given a tour of my friend's dairy, I enlisted his sister’s help in performing an blumpkin decker in his bathroom.
by porkchopbiscut October 25, 2004
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Also referred to as 'Going Up Top,' the act of taking a shit in the tank of a toilet. Common courtesy not to flush. Used TP should be placed inside drawers, magazines, or the tank itself. Feel free to throw in any other items you can find in the bathroom during the process.
"Man this party sucks... I'll let 'em know how I feel with an Upper Decker"

"Dude!! You shit in my toilet" -> "Where else did you want me to go?" -> "Not in the top part you fuck!!"
by ShitBrick February 25, 2004
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