After Romeo complains that he is “done,” Mercutio makes a pun on the words “done” and “dun,” the color brown — “You’re not done, a mouse is dun.” Back when most houses still had brown earthen floors, a brown mouse was barely noticeable. A policeman (or a school teacher for that matter) would use the expression “Dun’s the mouse” to tell someone to stop causing a commotion and drawing attention to themselves.