shit and fall backwards in it
A response trivialising someone's description of what they plan to do, when the listener finds it extravagant or believes it won't actually be carried out or won't succeed.
Person A: My sister is so upset with her local councilman that she's starting a petition to have him recalled.
Person B: Yeah, she's gonna shit and fall backwards in it.
Person B: Yeah, she's gonna shit and fall backwards in it.