But as a proper noun, the -ism was coined by Thomas Huxley with a very specific definition as Michael Martin writes in "Atheism: A Philosophical Justification":
Putting aside the current popular sense of the term, "Agnosticism" was coined by T. H. Huxley in 1869. According to Huxley, Agnosticism is not a creed but a method, the essence of which lies in the vigorous application of a single principle. Positively the principle may be expressed as, in matters of intellect, follow your reason as far as it can carry you without other considerations. And negatively, in matters of the intellect, do not pretend conclusions are certain that are not demonstrated or demonstrable. It is wrong for a man to say he is certain of the objective truth of a proposition unless he can produce evidence which logically justifies that certainty.
As you can see from this definition Agnosticism is not just a statement about knowledge whereas atheism is a statement about belief (but the two positions can still be compatible). Agnosticism, properly understood is a Positive belief in what we might call science or skepticism. In this sense it is a very different thing from the common form of atheism today (which is a rejection of theistic beliefs, or more accurately "a lack of belief in a god or god(s)").
I personally believe that even Positive Atheism is a fairly well supportable position these days. 100% absolutely certainty is NOT the point of belief - you can reasonably believe something when it is not contradicted by the evidence and has reasonable supporting facts. For example, scientists generally believe that there are Black Holes even though they are far from absolutely proven to exist (as described by relativity) and there are many open questions.
In his paper "Am I An Atheist Or An Agnostic?" Bertrand Russell wrote:
Here there comes a practical question which has often troubled me. Whenever I go into a foreign country or a prison or any similar place they always ask me what is my religion.
I never know whether I should say "Agnostic" or whether I should say "Atheist". It is a very difficult question and I daresay that some of you have been troubled by it. As a philosopher, if I were speaking to a purely philosophic audience I should say that I ought to describe myself as an Agnostic, because I do not think that there is a conclusive argument by which one prove that there is not a God.
On the other hand, if I am to convey the right impression to the ordinary man in the street I think I ought to say that I am an Atheist, because when I say that I cannot prove that there is not a God, I ought to add equally that I cannot prove that there are not the Homeric gods.
None of us would seriously consider the possibility that all the gods of homer really exist, and yet if you were to set to work to give a logical demonstration that Zeus, Hera, Poseidon, and the rest of them did not exist you would find it an awful job. You could not get such proof.
Therefore, in regard to the Olympic gods, speaking to a purely philosophical audience, I would say that I am an Agnostic. But speaking popularly, I think that all of us would say in regard to those gods that we were Atheists. In regard to the Christian God, I should, I think, take exactly the same line.
From this, I think that in Russell's day a more positive atheism was the prevailing brand of atheism whereas today the majority base their position on the lack of evidence for theism.
I generally identify myself as Huxley-Agnostic these days but I do not believe in any gods and therefore I am, in that sense, atheistic and will sometimes use that term when discussing with less discerning company. But I have always disliked the idea of defining my position in opposition to theism as I don't place that much importance on it.
Certainly Agnostics (and many others) have pondered if we can even ask questions like "does god exist" or "what is the origin of the universe". Such things might be truly unknowable -- but I would want that claim to be demonstrated before I accept it as fact!