to play dummy

Dummy \Dum"my\, n.; pl. {Dummies}.
   1. One who is dumb. --H. Smith.
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   2. A sham package in a shop, or one which does not contain
      what its exterior indicates.
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   3. An imitation or copy of something, to be used as a
      substitute; a model; a lay figure; as, a figure on which
      clothing is exhibited in shop windows; a blank paper copy
      used to show the size of the future book, etc.
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   4. (Drama) One who plays a merely nominal part in any action;
      a sham character.
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   5. A thick-witted person; a dolt. [Colloq.]
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   6. (Railroad) A locomotive with condensing engines, and,
      hence, without the noise of escaping steam; also, a dummy
      car.
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   7. (Card Playing) The fourth or exposed hand when three
      persons play at a four-handed game of cards.
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   8. A floating barge connected with a pier. --Knight.
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   {To play dummy}, to play the exposed or dummy hand in cards.
      The partner of the dummy plays it. Dumose