Definition of fly noun from the Oxford Advanced American Dictionary

     

    fly

     noun
    noun
    NAmE//flaɪ//
     
    (pl. flies)
     
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    insect
  1. 1[countable] a small flying insect with two wings. There are many different types of flies A fly was buzzing against the window. Flies rose in thick black swarms.
  2. in fishing
  3. 2[countable] a fly or something made to look like a fly, that is put on a hook and used as bait to catch fish fly-fishing
  4. on pants
  5. 3[singular] an opening down the front of a pair of pants that fastens with a zipper or buttons and is usually covered over by a strip of material Your fly is open!
  6. on tent
  7. 4[countable] a piece of material that covers the entrance to a tent see also flies
  8. Idioms
    die/drop/fall like flies(informal)
     
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    to die or fall down in very large numbers People were dropping like flies in the intense heat.
    a/the fly in the ointment
     
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    a person or thing that spoils a situation or an occasion that is fine in all other ways
    a fly on the wall
     
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    a person who watches others without being noticed I'd love to be a fly on the wall when he tells her the news. fly-on-the-wall documentaries (= in which people are filmed going about their normal lives as if the camera were not there)
    (there are) no flies on somebody(informal)
     
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    the person mentioned is smart and not easily tricked
    not hurt a fly
     
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    to be kind and gentle and unwilling to cause unhappiness
    on the fly(informal)
     
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    if you do something on the fly, you do it quickly while something else is happening, and without thinking about it very much
See the Oxford Advanced Learner's Dictionary entry: fly