The specific taste left in ones mouth after a large night out drinking, smoking and general partying. The general characteristics are hairy feeling teeth and a bad, lingering taste of cigarettes and alcohol that refuses to go away no matter what you try. Other terms include cotton-mouth and hairy teeth
Person 1: #%$@ man! I got such bad Hang Mouth!
Person 2: Nasty! Why, you have a big night last night?