A millihelen is the amount of beauty required to launch a single ship. From Helen of Troy, "the face that launched a thousand ships." The problem with this is that it only takes into account the beauty of her face and not her mind or the rest of her body.
See The Iliad for a much more detailed description of all of the aforementioned events.
The first recorded use of millihelen is in the fortune file present on many *nix systems, in the late 1980s.