<jargon> 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).
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Nearby terms: proof « PROOF/L « proof theory « propeller head » propeller key » Proposal Writing » proposition