Walter Mitty is a fictional character in James Thurber's short story "The Secret Life of Walter Mitty", first published in The New Yorker on March 18, 1939, and in book form in My World--and Welcome to It in 1942. Danny Kaye starred in the movie.

Mitty is a meek, mild man with a vivid fantasy life: in a few dozen paragraphs he imagines himself a wartime pilot, an emergency-room surgeon, and a devil-may-care killer. The character's name has come into more general use to refer to an ineffectual dreamer, appearing in several dictionaries. The American Heritage Dictionary defines a Walter Mitty as "an ordinary, often ineffectual person who indulges in fantastic daydreams of personal triumphs."

From Wikipedia