When someone is so sure of their answer, they are willing to drink a shot glass full of mustard if they are incorrect. Usually used to overrule a friend or teammate's answer to a trivia question.
Roscoe was Mustard Shot Positive that the answer to the trivia question was "November Rain". He ended up drinking 1.5 ounces of dijon.
A small preliminary shit that, once expelled, gives way to a large volume of feces. It resembles and reminds one of the crust of dried mustard that forms on the cap of the bottle which must be voided before the main mass of the condiment can be squeezed out.
Jason: "How's your stomache ache, bro?"
Matt: "It's gone now. I had a thick mustard plug that was blocking a giant diarrhea bomb."
Jason: "Gross dude!"