T H Huxley coined the term "agnostic" as an antonym of "gnosis," from the Greek term for "knowledge." Some early Christian groups referred to themselves as Gnostics because they felt they had special, secret knowledge. (Keep this is mind because it makes the rest of this post even funnier.) Gnostics lost out in power and influence to other Christian groups who asserted that the knowledge of Jesus and His salvation was meant for all mankind, not just for an elite group.
Here is Huxley's account of how he came up with the term "agnostic."
When I reached intellectual maturity, and began to ask myself whether I was an atheist, a theist, or a pantheist; a materialist or an idealist; a Christian or a freethinker, I found that the more I learned and reflected, the less ready was the answer; until at last I came to the conclusion that I had neither art nor part with any of these denominations, except the last. The one thing in which most of these good people were agreed was the one thing in which I differed from them. They were quite sure that they had attained a certain "gnosis"--had more or less successfully solved the problem of existence; while I was quite sure I had not, and had a pretty strong conviction that the problem was insoluble. And, with Hume and Kant on my side, I could not think myself presumptuous in holding fast by that opinion ...
What the Hell is "the problem of existence"? Have you ever heard an atheist claim that he or she had solved it, even more or less? Yeah, me neither. Do you see a difference between atheists and freethinkers anywhere except in bizarre descriptions of them by agnostics, like this one? Yeah, me neither.
If there are any atheists who are completely sure that no God or god exists and would not change their minds on the subject if shown convincing evidence that He or She or they exist, they are a tiny minority of the group of atheists, statistically insignificant and sharply at odds with the rest of us on this point.
And that's just one point, one topic, one question, the question of the existence of God or gods. Whatever "the question of existence" might be, I'm having an awful time imagining it being something other than a question, a topic, which is far, far broader than the question of God or gods. (For the rest of this post, whenever I write "God," just assume I mean "God or any gods.")
I can think of some people whom the term "agnostic" sensibly fits. I'm not thinking of Huxley nor of most of the self-identifying agnostics, but of people who sometimes think that God exists and sometimes think he doesn't. I believe that some or all of the people who call themselves believers who have had "crises of faith" come under this category. In addition, there are some people who simply aren't sure at all, who could go one way or the other, either to atheism or theism.
But Huxley and most of the people who identify as agnostics don't think of God's existence as being more likely than do we atheists. The whole agnostic movement isn't about theology, it's about misuse of semantics and claims of intellectual superiority. Bertrand Russell illustrated this very nicely with his example of the teapot in space, orbiting the Sun somewhere between Earth and Mars. That teapot is like God: Russell couldn't refute its existence, and didn't need to; although if he meant this as a refutation of the supposed special status of agnosticism, he was much too nice to say so directly. (Luckily, I am not.) We do not know, any of us, with absolute certainty, that God doesn't exist. However, that does not say anything about God, or about any unique or special or even unusual properties which God may possess. On the contrary, God is exactly the same as everything else in this respect, because we do not know anything at all with absolute certainty. Nevertheless, we do not go around constantly wasting our time emphasizing that we cannot disprove the existence of that teapot in space, or of 10-foot-tall man-eating parrots just out of our sight, or of Bigfoot. And it takes an agnostic of the Huxley variety not to understand that God is no different than any other absurd proposition in this respect.
Agnostics like to think -- and very much want you to think, that they're -- you know, not like everybody says, like... stupid! They're smart! And they want some respect!
Well, Sparky, respect is a give-and-take thing. It has to be earned. Some people give no respect whatsoever to strangers. Most of us are a little more trusting and give everybody a little respect on first meeting, on credit, just to help things go. But you can go through that credit pretty quickly. You want respect? Give it. Agnostics, if you want the respect of atheists, go to the trouble of learning what we actually do and don't believe, before you start telling us what we're all about. Hear what we believe in our own words. Not in the words of your fellow agnostics. I cannot emphasize that strongly enough. If you're listening, you'll see that when it comes to the question of God, we believe what you do. You'll see that you're just one more of this vast group of atheists out here in the very general public, and that if you want to be more special than all of us, you'll have to do it some other way.
Except of course that you won't see any of this, because it's all so simple and clear that anyone who wants to see it already has, without any help from me. So you go right on thinking that you're much smarter than all of us, and that the only reason people like me verbally abuse you is because we're jealous cause you're so cool.
And for everybody else, my advice about T H Huxley is to read Mark Twain instead.