Source : Free On-Line Dictionary of Computing

propeller head
     
         Used by hackers, this is synonym with {computer
        geek}.  Non-hackers sometimes use it to describe all techies.
        Probably derives from SF fandom's tradition (originally
        invented by old-time fan Ray Faraday Nelson) of propeller
        beanies as fannish insignia (though nobody actually wears them
        except as a joke).
     
        [{Jargon File}]
     
        (1995-01-18)