Definition of why adverb from the Oxford Advanced Learner's Dictionary

      

    why

     adverb
    adverb
    BrE BrE//waɪ//
     
    ; NAmE NAmE//waɪ//
     
     
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  1. 1  used in questions to ask the reason for or purpose of something Why were you late? Tell me why you did it. ‘I would like you to go.’ ‘Why me? (informal) Why oh why do people keep leaving the door open?
  2. 2  used in questions to suggest that it is not necessary to do something Why get upset just because you got one bad grade? Why bother to write? We'll see him tomorrow.
  3. 3  used to give or talk about a reason That's why I left so early. I know you did it—I just want to know why. The reason why the injection needs repeating every year is that the virus changes. Express YourselfGiving reasons, justifying a choiceIn various exams, you are asked to make a choice and give reasons for it. In conversation or in a meeting, you need to explain and justify your decisions: There are two main reasons why I think it’s the best option: first, there's the cost and second, the quality. I think/​believe it's the right thing to do because it gives everyone a fair chance. I would choose the newer one on the grounds that it will last longer. Of the three houses, the largest one seems to me to be the best, because they need the room. My choice would be number 3, simply because it's the clearest design.
  4. Word Origin Old English hwī, hwȳ ‘by what cause’, instrumental case of hwæt ‘what’, of Germanic origin.Idioms used in questions to mean ‘why’, expressing surprise Why ever didn't you tell us before?  used to make or agree to a suggestion Why not write to her? ‘Let's eat out.’ ‘Why not? Why don't we go together?
See the Oxford Advanced American Dictionary entry: why