"How now, a rat?" is a famous line from William Shakespeare's play Hamlet [Hamlet, Act III, Scene iv]. In the scene, Hamlet is confronting his mother, Gertrude, about her marrying his uncle Claudius shortly after his father's death. He suspects someone is hiding behind a tapestry and, mistaking them for a rat, stabs through it.
Is there anything else you'd like to know about Hamlet or Shakespeare?