A BMW enthusiast. Comes from the fact that the BMW logo was inspired by a spinning airplane propeller, which dates back the BMW's beginnings as an aircraft manufacturer.
Non-geeks use this term in refrence to geeks. Geeks use this term in refrence to a geek that spews crap that he/she doesn't really know anything about.
non-geek: That guy has a Linux pda, what a propellerhead
geek: That propellerhead is vomiting random termanology in my face.