1. 
[syn: idiot, imbecile, cretin, moron, changeling, half-wit, retard]
2.  a child secretly exchanged for another in infancy; 
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.
      [1913 Webster]
            The changeling [a substituted writing] never known.
                                                  --Shak.
      [1913 Webster]
   2. A simpleton; an idiot. --Macaulay.
      [1913 Webster]
            Changelings and fools of heaven, and thence shut
            out.
      [1913 Webster]
            Wildly we roam in discontent about. --Dryden.
      [1913 Webster]
   3. One apt to change; a waverer. "Fickle changelings."
      --Shak.
      [1913 Webster]
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.]
      [1913 Webster]
            Some are so studiously changeling.    --Boyle.
      [1913 Webster]
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
Moby Thesaurus II by Grady Ward, 1.0:
58 Moby Thesaurus words for "changeling":
   agent, alternate, alternative, analogy, backup, change, comparison,
   copy, counterfeit, deputy, double, dummy, elf child, equal,
   equivalent, ersatz, exchange, fake, fill-in, ghost, ghostwriter,
   imitation, locum tenens, makeshift, metaphor, metonymy,
   next best thing, personnel, phony, pinch hitter, proxy, relief,
   replacement, representative, reserves, ringer, second string,
   secondary, sign, spares, stand-in, sub, substituent, substitute,
   substitution, succedaneum, superseder, supplanter, surrogate,
   symbol, synecdoche, third string, token, understudy,
   utility player, vicar, vice-president, vice-regent