Jackwhole
Someone who's so dumb they think they're smart and still manage to mess everything up.
My brother is a jackwhole. He tried to fix the TV and now it's broken and he's still bragging.
My friend's dad is a jackwhole. He thinks he's a superhero and still can't tie his shoes.
My pet goldfish is a jackwhole. He won't eat my cereal and keeps swimming in my coffee.