An androgynous person or fictional character (human or other being) whom the outer appearance, voice and mannerisms conflict with gender-specific norms. Usually applied in reference to male anime characters who look and even sound like females. Slippy is a reference to the androgynous character Slippy in StarFox, whose gender to this day remains unknown.
Person 1 - "Ok I'm confused. Is that a chick or a dude? I don't know whether to be attracted or ashamed of myself."
Person 2 - "I have no idea, lets call it a slippy until we find out for sure on Wikipedia."