shit in my cherrios
When you ruin somebody's day by telling them some really bad news or make them do something really horrible, usually the receiving person is in a very good mood before said action happened.
person 1: I just won a scratch off ticket.
person 2: Your dog just got hit by a car.
person 1: Man, you just shit in my cherrios!
person 2: Your dog just got hit by a car.
person 1: Man, you just shit in my cherrios!