Definition of why adverb from the Oxford Advanced American Dictionary

      

    why

     adverb
    adverb
    NAmE//waɪ//
     
     
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  1. 1used 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. 2used 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. 3used 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.
  4. 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 Learner's Dictionary entry: why