The Collaborative International Dictionary of English v.0.48:

Why \Why\, adv. [OE. whi, why, AS. hw[imac], hw?, instrumental
   case of hw[=a], hw[ae]t; akin to Icel. hv[imac] why, Dan. &
   Sw. hvi; cf. Goth. hw?. ?. See Who.]
   [1913 Webster]
   1. For what cause, reason, or purpose; on what account;
      wherefore; -- used interrogatively. See the Note under
      What, pron., 1.
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            Turn ye, turn ye from your evil ways; for why will
            ye die, O house of Israel?            --Ezek.
                                                  xxxiii. 11.
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   2. For which; on account of which; -- used relatively.
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            No ground of enmity between us known
            Why he should mean me ill or seek to harm. --Milton.
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            Turn the discourse; I have a reason why
            I would not have you speak so tenderly. --Dryden.
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   3. The reason or cause for which; that on account of which;
      on what account; as, I know not why he left town so
      suddenly; -- used as a compound relative.
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   Note: Why is sometimes used as an interjection or an
         expletive in expression of surprise or content at a
         turn of affairs; used also in calling. "Why, Jessica!"
         --Shak.
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               If her chill heart I can not move,
               Why, I'll enjoy the very love.     --Cowley.
         [1913 Webster] Sometimes, also, it is used as a noun.
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               The how and the why and the where. --Goldsmith.
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   For why, because; why. See Forwhy. [Obs. or Colloq.]
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The Collaborative International Dictionary of English v.0.48:

Why \Why\, n.
   A young heifer. [Prov. Eng.] --Grose.
   [1913 Webster] Whydah bird

WordNet (r) 3.0 (2006):

why
    n 1: the cause or intention underlying an action or situation,
         especially in the phrase `the whys and wherefores' [syn:
         why, wherefore]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "why":
   Chinese puzzle, argument, baffling problem, brain twister,
   conundrum, crossword puzzle, crux, enigma, enigmatic question,
   excuse, explanation, floorer, for which, for why, from what cause,
   ground, how come, jigsaw puzzle, knot, knotty point, mind-boggler,
   mystery, mystification, nut to crack, on what account,
   perplexed question, perplexity, poser, pourquoi, pretense, pretext,
   problem, proof, puzzle, puzzlement, puzzler, question,
   question mark, rational ground, rationale, reason, reason for,
   reason why, riddle, sixty-four dollar question, stated cause,
   sticker, stumper, the big idea, the idea, the whatfor,
   the wherefore, the why, tough proposition, underlying reason,
   vexed question, what for, wherefore, whyever, whyfor