Polish words “powolny, powolna” have another older meaning. It means the work or activity performed according to one’s will or obedience. As far as I know Byelorussians and Ukrainians have similar word, and maybe the meaning is alike. You can also find in Poland a surname Powolny for men and Powolna for women which has no abusive nature at all.
So I think, that abusive meaning of Slovak world “povolna” can derive from describing the woman who… ahem… deals with man freely, not under coercion and probably has fun with it.