A blowhard is someone who always brags or boasts about himself. If you get stuck sitting next to your blowhard cousin at the family holiday dinner, you may wish he didn't think he was quite so amazing.
Blowhard is an informal word describing someone who can't stop talking about themselves or their accomplishments, real or imagined. You might know a blowhard at school, at work, or even in your family. And usually everyone knows at least one blowhard sports fan, who during the game can't stop talking about the unprecedented greatness of his favorite team or player.