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1.
a practical joke device consisting of a rubber air bladder that makes a flatulence sound when sat upon
If I sit on another whoopee cushion I'll have to play a prank on they'll never forget.
by The Return of Light Joker September 14, 2009
 
2.
When someone is trying to fart in the face of another person, and they splatter diarrhea all over the inside of their pants.
"Would you rather have me Whoopee Cushion your sister, or . . . wha- wha- wait. No."
by A Friggen Moose April 09, 2010