the main character in a well-known story by James Thurber , The Secret Life of Walter Mitty (1942). He escapes from his sad, ordinary life by imagining that he is a much more interesting person than he really is and does much more exciting and important things than he actually does. The name is now sometimes used of a person who lives in an imaginary world like this:
He’s a real Walter Mitty character, daydreaming the whole time.