What is "to play dummy"

Dummy \Dum"my\, n.; pl. Dummies.

1. One who is dumb.
   --H. Smith.

2. A sham package in a shop, or one which does not contain what its exterior indicates.

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.

4. (Drama) One who plays a merely nominal part in any action; a sham character.

5. A thick-witted person; a dolt. [Colloq.]

6. (Railroad) A locomotive with condensing engines, and, hence, without the noise of escaping steam; also, a dummy car.

7. (Card Playing) The fourth or exposed hand when three persons play at a four-handed game of cards.

8. A floating barge connected with a pier.
   --Knight.

   To play dummy, to play the exposed or dummy hand in cards. The partner of the dummy plays it.