1. Arrogant or pompous. Basically a person who likes to talk themselves up, in an unsuccessful attempt to be cooler.
2. Flaky; unreliable or a liar.
1. "Did you hear John talking about how he beat up four guys by himself??"
"Yeah... what an eatass."
2. "John said he'd come with us this time, but he's late again... the eatass."