It used to be that a "dude" was a guy. And only a guy. For instance, once upon a time it would have been appropriate to say, "Dude, Viagra can help that," but not "Dude, are you having your period?" As another example, everybody's favorite grammarians, Aerosmith, once wrote a song called "Dude Look Like a Lady." And this was noteworthy, and worth singing about and writhing on stage over, because most dudes did not.
But now, from what I can tell from my hipster/indie friends who are up on the latest slang, "dude" is a cross-gender noun meaning "person." When did this happen? And why? And what word has replaced the formerly serviceable "dude" to mean "generic male slacker"?