An offensive adjective <- usually -> refering to a female, hopefully at least 15 and it doesn't matter after 29, who is not only brazenly promiscous, but also indiscriminately so. However, nowadays, slutty can refer to any young woman "sowing her wild oats" or maybe that should be "having her wild oats sowed?"
That 15 year old slutty Lolita in high school, who panty-less does beaver shots during homeroom:
Guy1: "I'm dating The Slut on Saturday."
Guy2: "Well, you'll be doing sloppy seconds after my date on Friday."
Guy3: "Hey Guys, guess what? I'm tooling the slut Friday during lunch period!"
Shortly before a first marriage, the 29 year old woman is asked by her soon to be husband, "I know you are no virgin and Bob wanted to marry you for years, but how many guys did you share sex (and is my little dick big enough)?"
The ethical slut replies: (I did the basketball teams, both high school & college, pulling a train) "Soulmate, Bob got my virginity and boy, was he clumsy! Not like you." (And who cares that you have a stubby pencil dick, when I can hit the Hitachi?) (You do have the most enthusiastic tongue.) "Besides, you know how to please a woman and I don't want to know how you learned that!"