A dirty, slutty, whore of a woman. Loose refers to the elasticity of her oft-used vagina.
(At your friend's wedding, a hot, yet obviously slutty girl is on the dance floor, dropping it to the floor and flashing everyone her vag)
Kai: Yo, did you see that girl in the black dress?
Drew: With the brown hair? You mean Jessica. I know her from college.
Kai: Oh... Is she Loose McGoose?
Drew: I mean, she's been known to suck a dick or four...