An atheist who goes out of his or her way to argue about religion and treats all religious people with disrespect. This is a common phase which newly atheistic people go through. Those who remain Asshole Atheists after the fact may be generally antisocial or using atheism to compensate for a lack of other interesting traits.
Asshole Atheists generally give other atheists a bad name because others associate atheism with them rather than those who don't point out their atheism when not provoked.
Theist: You atheists are so obnoxious, Noah just bitched me out for saying "God bless you" when he sneezed.
Atheist: Dude, he's just an Asshole Atheist, don't associate the rest of us with him.