noun: An ordinary, timid person who indulges in daydreams involving great adventures and triumphs.
After the title character in The Secret Life of Walter Mitty, a short story (1939) by James Thurber, later made into a movie (1947) of the same name.
James Thurber’s story appeared in the March 18, 1939 issue of the New Yorker. In the story, Walter Mitty is a meek husband, rather uxorious, who fantasizes of great exploits to escape the humdrum of daily life. One minute he is dreaming of being a heroic pilot ("Throw on the power lights! Rev her up to 8500!"), next minute he becomes a daring naval commander. In his next thought he transforms into a master surgeon, and even a cool killer.
"It was not a Mitty dream. It was no fantasy at all."
Richard Bach; A Gift of Wings; Dell; 1974.