It is used to describe a very ugly and unpleasant girly figure. Used to describe girls aging between 16-27. It is used as if you were expecting better from the subject in terms of beauty and performance.
-- Oh dude... totally... she's a minge... (thrown in a normal ‘bollocs chat’ with your work mates)
-- She's a fucking Minger! (Harassing)
-- U fuckin' minge. (said in a really bad manner with a dose of hate, maybe thrown in 'talking to yourself style' while trying to get on the tube in rush hour)