of a James Thurber
story called, strangely enough, "The Secret Life of Walter Mitty
". Mitty is a small, unsuccessful Milquetoast
of a man, given to vivid daydreams about other lives he might have led if he had the willpower to at least stand up to his overbearing wife.
I'm sure this story still appears somewhere in the curriculum of every English Lit class in American schools, and is probably still much appreciated by the kids who like those classes anyway, being as it is a paean to nerdy fantasies.
The name Walter Mitty was, for several decades at least, used to refer to anyone who exhibited similar traits, probably because of the near-universal empathy that most readers feel for the poor guy.