As I use the term, it's not about being permanently horny (which, as thir pointed out, can be a horrible disability), but about being highly responsive. When I have my pet warmed up till she starts to beg with her hips, and squirms against me without needing to be told, then I say "that's my slut!"

In compounds it indicates someone who can't get enough of the thing in question, as in painslut, cumslut etc.