The word geek is a slang term originally used to describe eccentric or non-mainstream people, with different connotations ranging from an expert or enthusiast to a person heavily interested in a hobby, with a general pejorative meaning of a peculiar or otherwise dislikable person, esp one who is perceived to be overly intellectual.

Also see the related genre: nerdcore
Nrdcore rappers rhyme about anything from politics to science fiction, there are some perennial favorites in nerdcore subject matter, including Star Wars, role-playing games, science, fantasy and computers.