Peter Pan (n.)

name of the boy-hero in J.M. Barrie's play "Peter Pan, or The Boy Who Wouldn't Grow Up" (1904), first introduced in Barrie's "The Little White Bird" (1902). Used allusively for an immature adult man from 1914 (by G.B. Shaw, in reference to the Kaiser).

Well, Peter Pan got out by the window, which had no bars. Standing on the ledge he could see trees far away, which were doubtless the Kensington Gardens, and the moment he saw them he entirely forgot that he was now a little boy in a nightgown, and away he flew, right over the houses to the Gardens. It is wonderful that he could fly without wings, but the place itched tremendously, and, perhaps we could all fly if we were as dead-confident-sure of our capacity to do it as was bold Peter Pan that evening. [Barrie, "The Little White Bird"]