In this film, a visual pun is made of this term when a fly cop proves to be a policeman who can fly.
We are informed that a 'fly cop' sometimes called a 'shadow bull' or a 'tec' means nothing more nor less than a 'plain clothes man,' a sleuth, detective.
Law Notes, Volume XX, April, 1916 to March 1917, Edward Thompson Law Company, Northport, Long Island, New York, 1917