Euphemism for vagina. Coined by Joy in the TV comedy "My Name Is Earl".

Joy: "We gotta work backwards. Like when you rewound that videotape to see if Sharon Stone really flashed her undersmile."
> With "flashed her undersmile", Joy is referring to the film Basic Instinct, specifically the scene where Catherine Tramell (Sharon Stone) distracts her interrogators by uncrossing her legs to reveal a lack of underwear.