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]

Moby Thesaurus II by Grady Ward, 1.0:

208 Moby Thesaurus words for "freak":
   Bohemian, LSD user, aberration, abnormal, abnormality, abortion,
   acidhead, addict, aficionado, alcoholic, amateur, androgyne,
   anomalous, anomaly, aspirant, aspirer, atypical, authority,
   baroque, beatnik, bee, bigot, bizarre, boutade, brainstorm, buff,
   bug, candidate, capriccio, caprice, chain smoker, character,
   chimera, cocaine sniffer, cokie, collector, conceit, connoisseur,
   coveter, crackpot, crank, craze, crazy idea, critic, crosspatch,
   crotchet, cubehead, curiosity, deformed, deformity, demon, desirer,
   deviant, devotee, dilettante, dipsomaniac, dissenter, dope fiend,
   doper, dropout, drug abuser, drug addict, drug user, drunkard,
   eager beaver, eccentric, eccentricity, energumen, enthusiast,
   erratic, exceptional, expert, extraordinary, fad, faddist, fan,
   fanatic, fanatico, fancier, fancy, fantastic notion, fantasy,
   fiend, flake, flimflam, flower child, fool notion, freak of nature,
   freakish, freakish inspiration, freaky, glue sniffer,
   great one for, grotesque, habitual, hankerer, harebrained idea,
   head, heavy smoker, heretic, hermaphrodite, hippie, hobbyist,
   hopeful, hophead, hound, humor, hype, idiosyncrasy, individualist,
   infatuate, inner-directed person, irregularity, junkie, kink, kook,
   lover, lunatic fringe, maggot, malconformation, malformation,
   malformed, maniac, marijuana smoker, maverick, megrim, methhead,
   misbegotten, miscreation, misfit, misshape, misshapen, monomaniac,
   monster, monstrosity, monstrous, mosaic, mutant, mutation,
   narcotics addict, naysayer, nonconformist, nonjuror, notion, nut,
   odd, oddball, oddity, original, passing fancy, peculiarity,
   pillhead, pothead, pundit, pursuer, queer, quirk, rara avis, rare,
   rarity, recluse, rhapsodist, rococo, savant, scholar, screwball,
   sectarian, sectary, snowbird, solicitant, specialist, speed freak,
   sport, strange, strange duck, sucker for, suitor, swinger,
   technical expert, technician, teratism, teratogenic, teratoid, toy,
   tripper, ugly duckling, unconformist, unexpected, unforeseen,
   unique, unparalleled, unpredictable, unpredicted, unusual, user,
   vagary, visionary, votary, wanter, weird, weirdo, whim, whim-wham,
   whimsy, wisher, yearner, yippie, zealot