The Collaborative International Dictionary of English v.0.48:

Knack \Knack\ (n[a^]k), v. i. [Prob. of imitative origin; cf. G.
   knacken to break, Dan. knage to crack, and E. knock.]
   1. To crack; to make a sharp, abrupt noise to chink. [Obs. or
      Prov. Eng.] --Bp. Hall.
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   2. To speak affectedly. [Prov. Eng.] --Halliwell.
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The Collaborative International Dictionary of English v.0.48:

Knack \Knack\, n.
   1. A petty contrivance; a toy; a plaything; a knickknack.
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            A knack, a toy, a trick, a baby's cap. --Shak.
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   2. A readiness in performance; aptness at doing a specific
      task; skill; aptitude; facility; dexterity; -- often used
      with for; as, a knack for playing the guitar.
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            The fellow . . . has not the knack with his shears.
                                                  --B. Jonson.
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            The dean was famous in his time,
            And had a kind of knack at rhyme.     --Swift.
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   3. Something performed, or to be done, requiring aptness and
      dexterity; a trick; a device. "The knacks of japers."
      --Chaucer.
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            For how should equal colors do the knack ! --Pope.
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WordNet (r) 3.0 (2006):

knack
    n 1: a special way of doing something; "he had a bent for it";
         "he had a special knack for getting into trouble"; "he
         couldn't get the hang of it" [syn: bent, knack, hang]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "knack":
   ability, adroitness, aptitude, aptness, art, bauble, bent, bibelot,
   bric-a-brac, capacity, command, dexterity, expertise, expertism,
   expertness, facility, feel, flair, fribble, gaud, genius, gewgaw,
   gift, gimcrack, hang, head, intuition, kickshaw, knickknack,
   know-how, mastership, mastery, nose, proficiency, quickness,
   readiness, set, skill, swing, talent, touch, toy, trick, trinket,
   turn, way, whim-wham