As said before, you are an agnostic atheist if you do not believe in the existence of God (atheism), but also do not claim to know that He doesn't exist (agnosticism).
However, I do find the term "agnostic atheist" to be superfluous. Just "atheist" is good enough.
All atheists are already agnostics by default because no one actually knows whether or not God exists. Maybe they THINK they know, or maybe they CLAIM to know, but they don't. And if God doesn't exist, then likewise, you can't ever know that for sure and thus you are still an agnostic. The only way to know that God doesn't exist would be to have an evidence for his non-existence. But, how can there be an evidence for the non-existence of something? That's not possible, is it?
I think that terms such as "strong atheist" and "weak atheist" are more accurate because they specify the probability that an atheist holds about the existence of God. The term "agnostic" seems unnecessary to me because it's obvious that they they don't know. If someone does claim to know that God doesn't exist, then his claim is still not true, so the term is useless, in my opinion.
The point is that, claiming to know or not know that God doesn't exist just indicates the strength of an atheist's non-belief, but he doesn't actually know what he claims to know.
However, I do find the term "agnostic atheist" to be superfluous. Just "atheist" is good enough.

All atheists are already agnostics by default because no one actually knows whether or not God exists. Maybe they THINK they know, or maybe they CLAIM to know, but they don't. And if God doesn't exist, then likewise, you can't ever know that for sure and thus you are still an agnostic. The only way to know that God doesn't exist would be to have an evidence for his non-existence. But, how can there be an evidence for the non-existence of something? That's not possible, is it?
I think that terms such as "strong atheist" and "weak atheist" are more accurate because they specify the probability that an atheist holds about the existence of God. The term "agnostic" seems unnecessary to me because it's obvious that they they don't know. If someone does claim to know that God doesn't exist, then his claim is still not true, so the term is useless, in my opinion.
The point is that, claiming to know or not know that God doesn't exist just indicates the strength of an atheist's non-belief, but he doesn't actually know what he claims to know.