The Collaborative International Dictionary of English v.0.48:
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
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. Dumose