Lackwag
A lackwag is a person who thinks they're the best but are just plain average. They're like a pizza with no cheese, still edible, but not worth the trouble.
He said he was the best at chess. Then he lost to a kid in third grade.
She called herself a queen. Then she got beaten up by a group of girls.
That guy said he could beat anyone in a fight. Then he ran away when the fight started.