Jackwhole
A person who's so full of themselves they think the sky is made of their face and everyone else is just there to worship them.
My brother is a jackwhole. He thinks he's a superhero and still can't do his math homework.
My friend is a jackwhole. He thinks he's the best at everything and still loses at video games.
My neighbor is a jackwhole. He thinks he's a rockstar and still sings in the shower at 3 a. m.