A woman who exerts no natural-selective effort in having sex.
This definition explains why there is no double standard in labeling a woman a "slut" and a man a "stud". A true slut does not put forward any effort in being selective about sexual partners.
A (female) slut is a woman who just spreads her legs. She is not considered reproductively desirable as her offspring will not be as strong. Men won't commit to her, as future offspring will almost certainly not be theirs.
Whereas a "male slut" would be a man who uses prostitutes or is a virgin: two examples of a man exerting no effort. This shows there is no double standard, as these men are equally ostracized.
A man who has sex with many women must put forward a *great* amount of effort to do so, and as such is called a "stud". Women and men alike wonder what he has to be so desirable (confidence, hung, rich, etc).
All Trixie did was hear a short sales pitch, and she led him to the men's bathroom for a blowjob. She is a slut.
All John did was pay $10, and she led him to the men's bathroom for a blowjob. He is a slut.
All Jack did was lead himself to the men's bathroom for a self-inflicted hand-job. He is a slut.