The Collaborative International Dictionary of English v.0.48:

Freak \Freak\, n. [Prob. from OE. frek bold, AS. frec bold,
   greedly; akin to OHG. freh greedly, G. frech insolent, Icel.
   frekr greedy, Goth. fa['i]hufriks avaricious.]
   1. A sudden causeless change or turn of the mind; a whim of
      fancy; a capricious prank; a vagary or caprice.
      [1913 Webster]

            She is restless and peevish, and sometimes in a
            freak will instantly change her habitation.
                                                  --Spectator.

   2. a rare and unpredictable event; as, the July snowstorm was
      a freak of nature.
      [PJC]

   3. an habitual drug user, especially one who uses psychedelic
      drugs.
      [PJC]

   4. an animal or person with a visible congenital abnormality;
      -- applied especially to those who appear in a circus
      sideshow.
      [PJC]

   Syn: Whim; caprice; folly; sport. See Whim.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Freak \Freak\ (fr[=e]k), v. t. [imp. & p. p. Freaked
   (fr[=e]kt); p. pr. & vb. n. Freaking.] [Akin to OE. frakin,
   freken, freckle, Icel. freknur, pl., Sw. fr[aum]kne, Dan.
   fregne, Gr. perkno`s dark-colored, Skr. p[.r][,c]ni
   variegated. Cf. Freckle, Freck.]
   To variegate; to checker; to streak. [R.]
   [1913 Webster]

         Freaked with many a mingled hue.         --Thomson.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Freak \Freak\, v. i.
   1. to react with irrationality or extreme emotion; to lose
      one's composure; -- often used in the phrase

   freak out.
      [PJC]

   2. to become irrational or to experience hallucinations under
      the influence of drugs; -- often used in the phrase

   freak out.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Freak \Freak\, v. t.
   1. to cause (a person) react with great distress or extreme
      emotion; -- often used in the phrase

   freak out.
      [PJC]

WordNet (r) 3.0 (2006):

freak
    n 1: a person or animal that is markedly unusual or deformed
         [syn: freak, monster, monstrosity, lusus naturae]
    2: someone who is so ardently devoted to something that it
       resembles an addiction; "a golf addict"; "a car nut"; "a
       bodybuilding freak"; "a news junkie" [syn: addict, nut,
       freak, junkie, junky]
    v 1: lose one's nerve; "When he saw the accident, he freaked
         out" [syn: freak out, freak, gross out]