A slut is a girl who sleeps with other guys, but won't have sex with you.
1. If a guy is pissed off that the attractive girl in high school won't even look at him, but has sex with other guys, he'll say "she is just a slut" to make himself feel less bad about missing out.
2. If a guy is dumped by a girl for another guy he'll call her a slut to make her feel bad, perpetuating the notion that she has betrayed an (imaginary) ongoing obligation towards him. This is an extension of an old warped idea that guys have rights of ownership over their girlfriends or wives.