Definition of fast adverb from the Oxford Advanced American Dictionary

     

    fast

     adverb
    adverb
    NAmE//fæst//
     
    (faster, fastest)
     
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    quickly
  1. 1quickly Don't drive so fast! How fast were you going? I can't go any faster. The water was rising fast. Her heart beat faster. (formal) Night was fast approaching. a fast-flowing stream Which Word?quick / quickly / fast Quickly is the usual adverb from quick:I quickly realized that I was on the wrong train. My heart started to beat more quickly. Quick is sometimes used as an adverb in very informal language, especially as an exclamation:Come on! Quick! They’ll see us!Quicker is used more often:My heart started to beat much quicker. The quicker I get you away from here, the better. Fast is more often used when you are talking about the speed that someone or something moves at:How fast can a cheetah run? Can’t you drive any faster? You’re driving too quickly.There is no word fastly.
  2. 2 in a short time; without delay Children grow up so fast these days. The United States is fast becoming a nation of fatties. The police said that they reacted as fast as they could.
  3. firmly
  4. 3firmly; completely Within a few minutes she was fast asleep (= sleeping deeply). The boat was stuck fast (= unable to move) in the mud. There is no noun related to fast. Use speed in connection with vehicles, actions, etc.;quickness is used about thinking.
  5. Idioms
    as fast as your legs can carry you
     
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    as quickly as you can
    hold fast to something(formal)
     
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    to continue to believe in an idea, etc. despite difficulties
    play fast and loose (with somebody/something)(old-fashioned)
     
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    to treat someone or something in a way that shows that you feel no responsibility or respect for them
    stand fast/firm
     
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    to refuse to move back; to refuse to change your opinions
    thick and fast
     
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    quickly and in large quantities Questions were coming at them thick and fast.
See the Oxford Advanced Learner's Dictionary entry: fast