Taking a dump into the toilet tank so when a person flushes the toilet they are astonished to see a turd come down.
Marlene was shocked at the Omega Mu House party when she flushed the toilet and a big nasty frat boy turd came down and she realized that she had been the victim of double decking.

(WARNING: Do not try this. You could easy break a toilet. )
by Zoro the Ass Bandit April 02, 2013
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Two people farting within a small space of each other
Damn bro I could barely breathe in the car when Dan and Samantha were double decking last night
by Lilbodega March 05, 2018
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