Green’s Dictionary of Slang

a computer hacker; thus a socially inept individual.

1991		E. Raymond New Hacker’s Dict. 288: 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).
2019		S.M. Jones Lives Laid Away [ebook] ‘The propeller-heads at NOAA’ll probably think they got a tenth of a second of space noise’.