Jackamoe
A person who is so dumb they could be the kid who failed every test and still thinks they're the smartest in the class.
My brother is a jackamoe. He got a D on his math test and still thinks he's the best.
My friend says I'm a jackamoe because I said 2+2=5.
My cousin is a jackamoe. He failed the science test and still thinks he's a genius.