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1.
to fill up your ass with air and slap someone with it letting out a fart in the process
shane dawson said it

dude she got whoopie slapped
by idonthaveapseudonym.0 January 11, 2011
 
2.
When you fill your butthole with air, slap someone with it and a small poo comes out.
Dude if you don't shut up I'm gonna whoopieslap yo' ass
by ShadowNam January 10, 2011