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.
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            We ground the justification of our nonconformity on
            dark subtilties and intricate quirks. --Barrow.
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   2. A fit or turn; a short paroxysm; a caprice. [Obs.] "Quirks
      of joy and grief." --Shak.
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   3. A smart retort; a quibble; a shallow conceit.
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            Some odd quirks and remnants of wit.  --Shak.
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   4. An irregular air; as, light quirks of music. --Pope.
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   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.
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   6. (Arch.) A small channel, deeply recessed in proportion to
      its width, used to insulate and give relief to a convex
      rounded molding.
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   Quirk molding, a bead between two quirks.
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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"