Labunchkin
A person who talks so much and brags so hard that you're half-convinced they're lying, but you still have to listen because they won't shut up.
My teacher's pet is a labunchkin. She says she's the smartest in the class. She failed math.
My friend's brother is a labunchkin. He said he can beat anyone in a fight. He got beaten by a kid.
My cousin's friend is a labunchkin. He said he's a superhero. He tripped over a chair.