Someone who appears to be innocent when it comes to sex, but what lies under this person's innocent appearance is a sexual freak. Could apply to "tame" looking people who are sexually wild, or a person who is into sexual kinks. Can apply to women and men, but used often to refer to women.
I brought this one girl home. I was afraid of what to do because she seemed really uninterested in being more than friends, but when I walked in on her completely naked in my room, I realised I had brought home an undercover freak.