The Collaborative International Dictionary of English v.0.48:

Quirk \Quirk\ (kw[~e]rk), n. [Written also querk.] [Cf. W.
   chwiori to turn briskly, or E. queer.]
   1. A sudden turn; a starting from the point or line; hence,
      an artful evasion or subterfuge; a shift; a quibble; as,
      the quirks of a pettifogger. "Some quirk or . . .
      evasion." --Spenser.
      [1913 Webster]

            We ground the justification of our nonconformity on
            dark subtilties and intricate quirks. --Barrow.
      [1913 Webster]

   2. A fit or turn; a short paroxysm; a caprice. [Obs.] "Quirks
      of joy and grief." --Shak.
      [1913 Webster]

   3. A smart retort; a quibble; a shallow conceit.
      [1913 Webster]

            Some odd quirks and remnants of wit.  --Shak.
      [1913 Webster]

   4. An irregular air; as, light quirks of music. --Pope.
      [1913 Webster]

   5. (Building) A piece of ground taken out of any regular
      ground plot or floor, so as to make a court, yard, etc.;
      -- sometimes written quink. --Gwilt.
      [1913 Webster]

   6. (Arch.) A small channel, deeply recessed in proportion to
      its width, used to insulate and give relief to a convex
      rounded molding.
      [1913 Webster]

   Quirk molding, a bead between two quirks.
      [1913 Webster]

WordNet (r) 3.0 (2006):

quirk
    n 1: a strange attitude or habit [syn: oddity, queerness,
         quirk, quirkiness, crotchet]
    2: a narrow groove beside a beading
    v 1: twist or curve abruptly; "She quirked her head in a
         peculiar way"

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "quirk":
   aberration, anamorphism, anamorphosis, aroma, asymmetry, attribute,
   badge, boutade, brainstorm, brand, buckle, cachet, capriccio,
   caprice, cast, cavil, character, characteristic, conceit,
   configuration, contortion, crank, craze, crazy idea, crookedness,
   crotchet, cut, detorsion, deviation, differentia, differential,
   disproportion, distinctive feature, distortion, dodge, earmark,
   eccentricity, fad, fancy, fantastic notion, fantasy, feature,
   figure, flavor, flimflam, fool notion, freak, freakish inspiration,
   gnarl, gust, habit, hallmark, harebrained idea, humor, idiocrasy,
   idiosyncrasy, imbalance, impress, impression, index, individualism,
   irregularity, jesuitism, keynote, kink, knot, lineaments,
   lopsidedness, maggot, mannerism, mark, marking, megrim, minauderie,
   mold, nature, notion, oddity, odor, particularity, passing fancy,
   peculiar trait, peculiarity, property, quality, quibble, quiddity,
   quillet, quip, quodlibet, savor, screw, seal, shape, shuffle,
   singularity, smack, specialty, stamp, taint, tang, taste, token,
   torsion, tortuosity, toy, trademark, trait, trick,
   trick of behavior, turn, twist, unsymmetry, vagary, warp, whim,
   whim-wham, whimsy, wrench, wrest, wring