Walter Mitty is a fictional character in James Thurber's short story "The Secret Life of Walter Mitty". 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 Secret Life of Walter Mitty" was first published in The New Yorker on March 18, 1939.
http://en.wikipedia.org/wiki/Walter_Mitty
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