The Collaborative International Dictionary of English v.0.48:

geek \geek\ (g[=e]k), n.
   1. A performer in a carnival, often presented as a wild man,
      who performs grotesquely disgusting acts, such as biting
      the head off a live chicken or snake.
      [PJC]

   2. Hence: Any eccentric or strange person; an oddball; an
      eccentric. [WordNet sense 1]
      [PJC]

   3. Hence: A student who is socially inept and a misfit in his
      class, especially one who is an intellectual; a nerd; a
      dork. [Informal]
      [PJC]

   4. Hence: An intellectually inclined person, especially one
      who is interested in scientific or technical subjects; as,
      a group of geeks wearing pocket protectors; -- originally
      a deprecatory and contemptuous term, but in the 1990's,
      with the increase in popularity of computers and the
      frequency of accumulation of great wealth by computer
      entrepreneurs, it has come to be used with noticeable
      frequency by technically competent people to refer to
      themselves, ironically and sometimes proudly. [Informal]
      [PJC]

WordNet (r) 3.0 (2006):

geek
    n 1: a carnival performer who does disgusting acts
    2: a person with an unusual or odd personality [syn:
       eccentric, eccentric person, flake, oddball, geek]

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

geek
 n.

    A person who has chosen concentration rather than conformity; one who
    pursues skill (especially technical skill) and imagination, not mainstream
    social acceptance. Geeks usually have a strong case of neophilia. Most
    geeks are adept with computers and treat hacker as a term of respect, but
    not all are hackers themselves ? and some who are in fact hackers normally
    call themselves geeks anyway, because they (quite properly) regard ?hacker?
    as a label that should be bestowed by others rather than self-assumed.

    One description accurately if a little breathlessly enumerates ?gamers,
    ravers, science fiction fans, punks, perverts, programmers, nerds,
    subgenii, and trekkies. These are people who did not go to their high
    school proms, and many would be offended by the suggestion that they should
    have even wanted to.?

    Originally, a geek was a carnival performer who bit the heads off chickens.
    (In early 20th-century Scotland a ?geek? was an immature coley, a type of
    fish.) Before about 1990 usage of this term was rather negative. Earlier
    versions of this lexicon defined a computer geek as one who eats (computer)
    bugs for a living ? an asocial, malodorous, pasty-faced monomaniac with all
    the personality of a cheese grater. This is often still the way geeks are
    regarded by non-geeks, but as the mainstream culture becomes more dependent
    on technology and technical skill mainstream attitudes have tended to shift
    towards grudging respect. Correspondingly, there are now ?geek pride?
    festivals (the implied reference to ?gay pride? is not accidental).

    See also propeller head, clustergeeking, geek out, wannabee, 
    terminal junkie, spod, weenie, geek code, alpha geek.


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

computer geek
computer nerd
geek
turbo nerd

    (Or "turbo nerd", "turbo geek") One who eats
   (computer) bugs for a living.  One who fulfils all the
   dreariest negative stereotypes about hackers: an asocial,
   malodourous, pasty-faced monomaniac with all the personality
   of a cheese grater.  The term cannot be used by outsiders
   without implied insult to all hackers; compare
   black-on-black usage of "nigger".  A computer geek may be
   either a fundamentally clueless individual or a proto-hacker
   in larval stage.

   See also Alpha Geek, propeller head, clustergeeking,
   geek out, wannabee, terminal junkie, spod, weenie.

   [Jargon File]

   (1997-06-26)