Definition of knock noun from the Oxford Advanced American Dictionary

     

    knock

     noun
    noun
    NAmE//nɑk//
     
     
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    at door/window
  1. 1the sound of someone hitting a door, window, etc. with their hand or with something hard to attract attention There was a knock on/at the door.
  2. hit
  3. 2 a sharp blow from something hard synonym bang He had a nasty knock on the head. Knocks and scratches will lower the value of valuable furniture.
  4. Idioms
    take a (hard, nasty, etc.) knock
     
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    to have an experience that makes someone or something less confident or successful; to be damaged Industry in the area has taken a knock with the closure of two factories. Her confidence took a further knock when she lost her job.
See the Oxford Advanced Learner's Dictionary entry: knock