The Collaborative International Dictionary of English v.0.48:

Changeling \Change"ling\, n. [Change + -ling.]
   1. One who, or that which, is left or taken in the place of
      another, as a child exchanged by fairies.
      [1913 Webster]

            Such, men do changelings call, so changed by
            fairies' theft.                       --Spenser.
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            The changeling [a substituted writing] never known.
                                                  --Shak.
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   2. A simpleton; an idiot. --Macaulay.
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            Changelings and fools of heaven, and thence shut
            out.
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            Wildly we roam in discontent about. --Dryden.
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   3. One apt to change; a waverer. "Fickle changelings."
      --Shak.
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The Collaborative International Dictionary of English v.0.48:

Changeling \Change"ling\, a.
   1. Taken or left in place of another; changed. "A little
      changeling boy." --Shak.
      [1913 Webster]

   2. Given to change; inconstant. [Obs.]
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            Some are so studiously changeling.    --Boyle.
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WordNet (r) 3.0 (2006):

changeling
    n 1: a person of subnormal intelligence [syn: idiot,
         imbecile, cretin, moron, changeling, half-wit,
         retard]
    2: a child secretly exchanged for another in infancy