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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.
by MKLynch May 11, 2011