The face you make during a song you really like, and you're dancing really slutty to. It probably involves pouting of the lips in an "o" like fashion, sticking your ass out and grinding like a cat in heat on the nearest person.

Slutface is the latin equivelant of the forename Marie. In ancient times it was used as an affection term of endearment for women who were extremely intelligent yet still had the morals of a sewer rat.