A girl/woman wearing short pants/skirt and/or revealing her belly, cleavage or som other parts of her boob is not a slut.
Trying to be sexy does not make a girl/woman a slut.
Using her body to get favours or get attention does not make her a slut, possibly just a bit skanky.
Having sex with many partners in a short time does not make a woman a slut.
However, having sex with many partners in a short time without having any fond feelings for any of them, kind of makes her a slut in my book.
It's not cool to use the word 'slut' for persons who aren't sluts for real. Because it you do, what do you call the real ones?