eat a bag of dick
You yell this when someone is so dumb it feels like you need to eat a whole bag of dicks. People usually yell it back at you in a never-ending argument.
My friend said I was ugly. I yelled, 'Eat a bag of dick!' He yelled it back at me.
My boss said I was lazy. I told him to eat a bag of dick. He said it back.
My sister called me a nerd. I said, 'Eat a bag of dick!' and she said it back.