Doing a Poopa Turdy is to pretend that you have 'accidentally' spilt hot chocolate over your lap and friend's bedsheets, when really you have just farted and 'accidentally' followed through.
*Whilst watching a film*
Girl 1: "Hey, err, what is that on your lap...?"
Girl 2: "Oh! It's just hot chocolate, I swear! She knocked it over me when she sat down!!"
Girl 3: "Are you sure? I think you might have done a Poopa Turdy."
Where one poops into a chalupa from Taco Bell then asks the guy behind the counter to eat it because their slogan is "if you arent satisfied, we'll eat it". After attempting this stunt and seeing that the person wont eat the chalupa, one might throw it against the back wall and yell "YEAH, I POOPED IN IT BITCH"