The Jargon File (version 4.4.7, 29 Dec 2003):

propeller head
 n.

    Used by hackers, this is syn. with geek. Non-hackers sometimes use it to
    describe all techies. Prob. 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).


The Free On-line Dictionary of Computing (30 December 2018):

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)