A point in time where one would be left with a proverbial (and occasionally literal) brick in the seat of their trousers.
"Hey, you know that one scene in Band of Brothers, in Bastogne, at the end of the artillery barrage, where Luz and Lipton have a dud shell hit about three feet away?"
"Now that's a brown pants moment if i ever saw one"
Take a hot, steamy shit in a bag, preferrably a brown bag, and then tie, or twist, it shut. Proceed to hit somebody with it, most likely the face so they can smell the steaming pile of shit.