Definition of thumb noun from the Oxford Advanced American Dictionary

      

    thumb

     noun
    noun
    NAmE//θʌm//
     
     
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  1. 1the short thick finger at the side of the hand, slightly apart from the other four She still sucks her thumb when she's worried.
  2. 2the part of a glove that covers the thumb There's a hole in the thumb.
  3. Idioms
    be all thumbs
     
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    to be awkward with your hands so that you drop things or are unable to do something
    a rule of thumb
     
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    a practical method of doing or measuring something, usually based on past experience rather than on exact measurement As a rule of thumb, you should cook a chicken for 20 minutes for each pound of weight.
    stand/stick out like a sore thumb
     
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    to be very noticeable in an unpleasant way The blue building stood out like a sore thumb among the whitewashed villas. If you wear a suit to the party, you'll stand out like a sore thumb.
    thumbs up/down
     
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    used to show that something has been accepted/rejected or that it is/is not a success Their proposals were given the thumbs down. It looks like it's thumbs up for their latest album. In contests in ancient Rome, the public put their thumbs up if they wanted a gladiator to live, and down if they wanted him to be killed.
      twiddle your thumbs
       
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    1. 1to move your thumbs around each other with your fingers joined together
    2. 2to do nothing while you are waiting for something to happen
    under somebody's thumb
     
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    (of a person) completely controlled by someone She has him under her thumb.
See the Oxford Advanced Learner's Dictionary entry: thumb