looked it up and paipan also refers to teenagers who have gone through puberty but havent grown pubic hair. basically a way to call someone childish.
it could also mean a whore/prostitute since in the edo period it was usually geisha/prostitutes in red light districts that would shave their genitals so that stigma has been passed down. if a female is seen with shaved genitals in a public bath, rumors might flow around if there are older people present.