A person who holds the view that the existence of a deity cannot be proven nor disproven, but personally leans towards the idea of there not being one. In this case, agnostic is an adjective. "Atheist" has to do with belief, and "agnostic" has to do with knowledge. Two completely different animals, but can be applied to each other. Basically, a person who doesn't know and doesn't believe.
Guy: So what are your religious views?
Agnostic atheist: I'm an agnostic.
Guy: So you're just a pussy atheist?
Agnostic atheist: No, you idiot. I'm an atheist, too. I'm an atheistic agnostic, or an agnostic atheist.
Some one who has agnostic beliefs and does not believe in the existence of any deities.
<Agnostic Atheist>~ I don't believe in god.
<Christian>~ Yeah, well what proof do you have that exemplifies his non-existence?
<Agnostic Atheist>~ There is no proof that proves God's existence, or non-existence.