why

[hwahy, wahy]
adverb
1.
for what? for what reason, cause, or purpose?: Why did you behave so badly?
conjunction
2.
for what cause or reason: I don't know why he is leaving.
3.
for which; on account of which (usually after reason to introduce a relative clause): the reason why he refused to go.
4.
the reason for which: That is why he returned.
noun, plural whys.
5.
a question concerning the cause or reason for which something is done, achieved, etc.: a child's unending hows and whys.
6.
the cause or reason: the whys and wherefores of a troublesome situation.
interjection
7.
(used as an expression of surprise, hesitation, etc., or sometimes a mere expletive): Why, it's all gone!

Origin:
before 900; Middle English; Old English hwī, hwȳ, instrumental case of hwæt what; cognate with Old Norse hvī


See reason.
Dictionary.com Unabridged

why's

[hwahyz, wahyz]
contraction of why is: Why's dinner so late?

See contraction.
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
why (waɪ)
 
adv
1.  a.  for what reason, purpose, or cause?: why are you here?
 b.  (used in indirect questions): tell me why you're here
 
pron
2.  for or because of which: there is no reason why he shouldn't come
 
n , whys
3.  (usually plural) the reason, purpose, or cause of something (esp in the phrase the whys and wherefores)
 
interj
4.  an introductory expression of surprise, disagreement, indignation, etc: why, don't be silly!
 
[Old English hwī; related to Old Norse hvī, Gothic hveileiks what kind of, Latin quī]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Etymonline
Word Origin & History

why
O.E. hwi, instrumental case (showing for what purpose or by what means) of hwæt (see what), from P.Gmc. *khwi (cf. O.S. hwi, O.N. hvi), from PIE *qwei, locative of *qwo- "who" (cf. Gk. pei "where"). As an interjection of surprise or to call attention to a statement, recorded from 1519.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
There are lots of explanations for why customer service is so bad.
Why the pursuit of happiness naturally includes melancholy.
We pie lovers demand to know why so many family restaurants no longer have
  cases full of fresh homemade pies.
Scientists don't know why some moments seem to last longer than others.
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