A saying popularized by moJoe, of impressive ambiguity and with many possible interpretations.

It could mean that she is a pre-op transexual, looking like a woman but actually still being a life support system for a penis.
It could mean that her (or his) gender is not necessarily what it may look like, and anyway it is NYFB.
It could also be just an attempt to epater le bourgeois.

Of course when you compound it with the following statement, namely "You may anger her with such genderfucking, and she will be forced to thwump you with her clittycock", you only get compounded obscurity plus mystery plus exciting hints.

And if you are the right kind of fetishist, just leave the keyboard and go masturbate somewhere quiet and private, will ya ?