A form of visual "hypnosis" used to make people think, at first glance and hopefully longer, that you are more attractive than you actually are through the use of:
-a great abundance of beautiful hair (either long, "big", or both).
The goal is to divert attention from one's plain or unattractive face, or unattractive body. This purposeful deception falls between "accentuating the positives" of one's appearance and actually undergoing cosmetic surgery or liposuction.
When Meredith thinks that her powers of hairnosis are fading, she goes to a salon and spends 150 dollars for a "recharge." However, once guys take a long, second look, the hairnosis is dispelled and they are not duped by the subterfuge.