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

      

    fast

     adverb
    adverb
    BrE BrE//fɑːst//
     
    ; NAmE NAmE//fæst//
     
    (faster, fastest)
     
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    quickly
  1. 1  quickly 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 somebody 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. Britain is fast becoming a nation of fatties. The police said that they had reacted as fast as they could.
  3. firmly
  4. 3  firmly; 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. Word Originadverb Old English fæst ‘firmly fixed, steadfast’ and fæste ‘firmly’, of Germanic origin; related to Dutch vast and German fest ‘firm, solid’ and fast ‘almost’. In Middle English the adverb developed the senses ‘strongly, vigorously’ (compare with run hard), and ‘close, immediate’ (just surviving in the archaic fast by; compare with hard by), hence ‘closely, immediately’ and ‘quickly’; the idea of rapid movement was then reflected in adjectival use. Extra examplesDon’t drive so fast! I can’t go any faster. He came round the corner blindingly fast.Idioms
    as fast as your legs can carry you
     
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    as quickly as you can
    (formal) to continue to believe in an idea, etc. despite difficulties
    play fast and loose (with somebody/something)
     
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    (old-fashioned) to treat somebody/something in a way that shows that you feel no responsibility or respect for them
    to refuse to move back; to refuse to change your opinions quickly and in large quantities Questions were coming at them thick and fast.
See the Oxford Advanced American Dictionary entry: fast