The term sl*t is used by males (and females, though with dif. contexts) to denigrate someone who has most of these qualities:
A. Does not have a penis. This is most important.
B. Is considered completely dominated by male control. Males are obsessed with control, but even more obsessed with futile attempts at resistance. It's "the chase."
C. Has no self-esteem. She obviously hates herself.
D. Has no self-control. She can't resist that manly charm.
Most usage of the word is somewhat like that above. There is a darker context though, and that is when sl*t means the following:
A. She loves sex, but she loves even more to deprive men of sex, or cut them short.
B. She is contemptuous, especially towards men.
C. She sleeps with some men just to spite other men
D. She sleeps with everyone but the "good guys" such as the accusatory male.
Both usages are fucking disgusting, but the second is downright predatory. The second usage of the word implies a deeply held hatred for women. You may not hear this hatred often, but that is because you probably aren't looking for it. This is what you hear from MRAs, rapists, angry ex's, and abusers of all sorts.
The bottom line is that this is about control. It has nothing to do with promiscuity. Promiscuity is great (worth 50 man points) and encouraged, as long as it's a man. When men say this out of pure hatred, it's usually because they believe the woman in question purposefully spited them. I see this a lot with angry ex-boyfriends and ex-husbands. Of course she must have been cheating on him. She hated him so much she had to sleep with every man she could find. He did everything right and then some. He was never abusive, psychotic, or authoritarian.
These men don't realize that they are the scum of the earth. They are more than delusional, they fucking sadistic and cruel. GTFuckingO if you meet someone like this. If contact with them is unavoidable, at least bring a gun.