The Collaborative International Dictionary of English v.0.48:

Dummy \Dum"my\, a. [See Dumb.]
   1. Silent; mute; noiseless; as a dummy engine.
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   2. Fictitious or sham; feigned; as, a dummy watch.
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   Dummy car. See under Car.
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The Collaborative International Dictionary of English v.0.48:

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

WordNet (r) 3.0 (2006):

dummy
    adj 1: having the appearance of being real but lacking capacity
           to function; "a dummy corporation"
    n 1: a person who does not talk [syn: dummy, silent person]
    2: an ignorant or foolish person [syn: dumbbell, dummy,
       dope, boob, booby, pinhead]
    3: a figure representing the human form
    4: a cartridge containing an explosive charge but no bullet
       [syn: blank, dummy, blank shell]
    v 1: make a dummy of; "dummy up the books that are to be
         published" [syn: dummy, dummy up]