I realize now that is a completely obvious question. Really, because I've been wondering about a term related to sociology, I should have known that I can trace the word's use to its origin through the theories, and because identity is a theory based on symbolic interactionism (in many cases, and the related theory, social identity theory is similar enough that it probably grew up along side identity theory though I haven't researched this enough to know for certain), it can't go back any farther than Mead (1930s), and is most strongly developed by Styker and Burke, in identity theory and social identity theory respectively, so it's probably something around the 50s to 60s. And everyone shakes their heads because they are disappointed that I missed something so completely obvious, or because they have no idea what I'm talking about. And I shake my head because I realized on Wednesday that I had read, in its entirety, the article I have by Styker and Burke, and not only had I forgotten all of the content, I'd forgotten that I had read anything more than a page or two.
Oh sociology, the science of describing that which seems obvious after it has been pointed out, and then proving that it is not only obvious but true.