Sometimes one hears believers and even other non-believers calling some atheists "fundamentalists". I think the use of that term with regard to atheists is not only inaccurate but offensive on several levels.
Here is the definition from dictionary.com:
1. (sometimes initial capital letter) a movement in American Protestantism that arose in the early part of the 20th century in reaction to modernism and that stresses the infallibility of the Bible not only in matters of faith and morals but also as a literal historical record, holding as essential to Christian faith belief in such doctrines as the creation of the world, the virgin birth, physical resurrection, atonement by the sacrificial death of Christ, and the Second Coming.
2. the beliefs held by those in this movement.
3. strict adherence to any set of basic ideas or principles: the fundamentalism of the extreme conservatives.
The first two definitions clearly have no application to atheists in any way shape or form. They do tell us, however, where this particular term came from: A religious movement of extreme rigidity in its numerous doctrines and principles.
Given that atheism doesn't have a set of doctrines and principles (and thus doesn't fit even the third definition of fundamentalism from dictionary.com) but rather has only one principle, then the use of such term regarding an atheist must always be inaccurate. Thus, one possible reply would be:
"Atheists can't be fundamentalists because, there are no atheist 'fundamentals' for us to adhere to. We simply don't believe in a god."
If atheists strictly adhere to one principle--that we think there is no god--that does not make us "fundamentalists"; it makes us atheists. But, what else could we possibly do? If we don't strictly adhere to that one principle, then we would not be atheists, we would be agnostics. Thus, at best, the use of "fundamentalist" is simply redundant.
We are being chastised for not having an "open mind" in circumstances where the evidence (or lack thereof) is in. As I have explained before, this can only be based on some sort of implicit assumption that we have to have absolute positive proof of god's nonexistence in order to be atheists and that we have to take seriously the possibility that solipsism is reality (that we live in a universe comparable to the one in "The Matrix" only with an invisible magic man rather than machines pulling our strings).
This is one of many reasons I find the use of the term with regard to atheists offensive. We simply can't win. Merely being atheists makes us fundamentalists by that "logic". You can say:
"If not being agnostic makes me an atheist fundamentalist, then you are saying that simply being an atheist makes me an atheist fundamentalist. That makes 'fundamentalist' redundant and therefore mere name calling."
In addition, having spent a great deal of time among true fundamentalists and having suffered at their hands, I find such name-calling highly offensive. Some atheists may find it amusing because most of them are smart enough to see that the term has no application to them.
"Sorry, fundamentalism is a christian religious movement. Simply insisting on a rational approach to the world doesn't begin to resemble the depth and breadth of their rigidity."
I also find it offensive because, when theists say that atheists are fundamentalists they are using that term incorrectly, which indicates they are simply trying to be offensive. Oftentimes trying to be offensive is enough to actually be offensive, even if you look a fool in the process. You can make that point by saying:
"You know that term doesn't apply; you are just trying to be offensive."
Such inaccurate name calling does not add anything to the discussion. At best, it is simply the spewing of venom. At worst, it is a deliberate attempt to shout the other person down--to cow him with the threat of being labeled with an epithet that carries a social stigma.
This is the most common purpose with which the "atheist fundamentalist" label is wielded. It is yet another attempt to protect religion from critical analysis through the use of social pressure--otherwise known as bullying. Thus, use of that term is little better than censorship, with a hint of a threat of social ostracism or worse for the person on the receiving end. For that reason alone, I would think that most atheists would find it offensive.
Furthermore, given that logically the only conclusion one can reach is that god doesn't exist, using a term like "atheist fundamentalist" is a bit like calling a chemist a "chemistry fundamentalist" because he thinks the notions of medieval alchemy were ridiculous. Or calling a psychiatrist a "sanity fundamentalist" because he thinks that people who think they are receiving message from outer space aliens through the fillings in their teeth are mentally ill. You can use these analogies to make that point:
"Is a chemist a 'chemistry fundamentalist' because he ridicules notions of alchemy?"
The principles that brought me to atheism are the same ones that drive science: Objectivity and rationality. If adhering to those makes me a "fundamentalist", then so be it. So far as I am concerned, that would be the same as calling me fundamentally sane. Once again, you can use this idea to drive the point home:
"Adhering to notions of objectivity and rationality make me a 'fundamentalist'? That's like calling me fundamentally sane."
Sometimes the epithet is meant to imply that atheists are at fault for not examining the "evidence" for theism, then I can only point out that there is no evidence to examine (other than the mountain of evidence tending to prove that religions are man made). I can also point out that atheists, along with many others, have been diligently searching for evidence to support theism and will continue to do so. Those same principles of objectivity and rationality require that this search continue for the indefinite future, but they do not require that we reserve judgment in the meantime.
Calling atheists fundamentalists is really just a euphemism for calling us intolerant. Like most instances of "name calling", such as calling us "intolerant", calling us fundamentalist is just another way of trying to pressure us into shutting up or adopting the idea that we have to have absolute proof that god doesn't exist.