eat that mule shit
The final punchline when you've run out of ideas and your brain is full of donkey droppings.
You tried to argue. You tried to laugh. Now you're just tired. 'Eat that mule shit.'
Your friend made a joke. You made a joke. Now you're both just annoyed. 'Eat that mule shit.'
You're too tired to think. You just say it. 'Eat that mule shit.'