Used to describe a woman whose behavior is so heinous, the mere use of slut or cunt is inadequate. Covers a variety of horrid behavior, notably that presented by characters in shows like "Desperate Housewives".
Usage varies. Typically, when one learns of a woman who has killed her husband or children, or plans to, one reacts simply by saying "slunt!" If sex is involved, the appelation is upgraded to "that fucking slunt!"