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.