A person that is known to rant and rave about things. Their opinions are usually ignorant and lack evidence. The term comes from the literal meaning of "blowing hard". If you blow hard enough people pretty much have to listen.
Famous blow-hards include Glenn Beck and Dr. Phil.
Someone who boasts excessively and exaggeratively about his or her imagined talents or abilities. Also characteristic of someone who thinks they know something that others do not. Blow-hards are akin to name-droppers.
"Ever since John started attending Yale University, he has become such a blow-hard."