eat a bag of dick
You say this when someone is so dumb they should eat a whole bag of dicks. You also say it when someone is so smart they should eat a whole bag of dicks too.
My teacher said I was smart. I said, 'Eat a bag of dick!' to her.
My friend said I was stupid. I told him to eat a bag of dick.
My boss said I was good. I said, 'Eat a bag of dick!' to him.