To poop in one's mouth is a Midwest American colloquial expression, meaning:
1. To poke fun at someone, similar to the English expression "taking the piss";
2. To utterly dominate an opponent, in the context of sports or games.
1. It was first and goal and I was sure Team A was going to score a touchdown, but Team B's defense decided to rush the QB and poop in his mouth, they stopped him no problem.
To whine and complain that you don't have much money, that you're destitute from other expenses etc. when negotiating for goods or services when, in reality, you're loaded and much better off than the person you're haggling with.