Mit ‧ ty, Wal ‧ ter /ˈmɪti, ˈwɔːltər/ BrE AmE
someone who seems very ordinary but who either imagines they have an exciting secret life or who actually does have one. This name, used especially in newspapers, comes from the main character in a story by James Thurber called The Secret Life of Walter Mitty (1932). He has a very ordinary life, but spends a lot of time imagining that he is a brave and important person living a dangerous and exciting life:
Their quiet neighbour turned out to be a Walter Mitty character, running a huge drug-smuggling business from his garage.