So, I know, you're expecting me to say something about the denigration of women, blah blah. 'Cept I'm really not. I'm going to talk about the history of the word, which may, incidentally, reveal some of my problems with the cultural perception of women in western society.
In the 1630s or '40s, the word 'slattern' started to show up, in reference to women. Probably related to the verb 'slatter', meaning to spill, slosh, or awkwardly splash. (Consider this may also be related to modern 'splatter'.) A slattern was a woman who was unkempt, unclean, or, well, slattered upon. Pretty simple, that.
Now, slut, slattern, and slatter are all from the same roots (Swedish, Dutch, and Low German words of similar meaning), and slut and slattern can be considered dialectical variances in the same word.
Let's get that down to a finer point. A slut is a woman who is physically unclean and does not take care of herself. In fact, Chaucer used the word 'sluttish' to describe a man's appearance, meaning he looked persistently rumpled and unwashed.
From there, it came to mean a woman who was brazen and possibly of loose morals. But, 'loose morals' were sort of implied of anyone who did not take care of their appearance. I'm pretty sure there's a biblical reference that I'm too lazy to look up right now. The fact that 'slut' skips right over actual prostitution and goes straight to a woman who freely provides sex means that it passes right by the slovenly conditions surrounding street-walkers and goes in for the idea that it's another failure not only of morals, but of sense and the ability to care for oneself.
... Say what?
Right, so consider that 'appropriate' kinds of sex for a woman to have were the kinds in which she traded it for something of value, whether food and shelter, in the case of a wife, or money, in the case of a prostitute. Transactional heterosexuality. So, the implied reasoning here is, if a woman hadn't the sense to keep herself clean and her clothes well-fit, then she hadn't got the sense not to give away the only thing of value she had.
Slut: it doesn't just mean you fuck for free. It means you're stupid and dirty, too.
Anyway, it's the sort of word that really shouldn't be applied at all, in the modern era, since we're slowly slaying the beast that is transactional heterosexuality, but if it must be applied, it surely shouldn't be applied to pop stars and middle-class college girls out for a good time. Word just doesn't fit. That free-loving dirt-punk down by the arts district? Yeah, she might be a slut, in the traditional sense, but she's probably also going to kick your ass to the kerb and stomp on it, for saying any such thing.
Thank you. You may now return to mangling the language.
[=EDIT=] Wow, my bad, 'slut' predates 'slattern' by more than 200 years. Sluttish, via Chaucer, circa 1386; slut circa 1402; slattern circa 1639. *coughs* Still, the point remains -- the words from which it evolved do mean dirty, unkempt, or possibly even muddy. The connotation of promiscuity seems to have arisen around 1450, for 'slut'.