Originally, a phrase coined by Freud to describe an idea that he.. well.. It's not easy to say this but..
Look Freud was kind of a nutbag, ok? Sure, he did some good stuff but a lot of was just bad science. [Ed.: that was actually extremely easy to say]
"Vagina dentata" is a classic example of Freud's twisting of facts. He tried to prove the idea that men were naturally afraid of their penises being eaten by vaginas.
He claimed that the male clients (or patients or however psychoanalysts call them.. can't remember) he had interviewed has all shared this fear. Of course, he extrapolated this supposedly common fear from the dreams they described to him. The problem is when you actually look at his data, it's quite clear he distorted the truth. Freud needed to talk to a professional badly (ha! irony! I win the "I'm such a good writer" contest! It's even better when you point it out!). The good doctor had a habit of coming up with bizarre theories that didn't fit his data, but instead seemed to emanate from his own personal fears and quirks. Of course, not everything he said was completely outside of reality but he had more than his share of strange theories popping up.
Now you can seem from the link, its been co-opted by a webcomic. I find it abso-fuckin'-lutely hilarious.
Did I mention its really late and I really need sleep? I become some weird being who spews nothing but inane comments after 0230. In order to make up for it, let me also point you to the a hand drawn digital clock. This one is courtesy of BoingBoing. Enjoy, folks.