The Collaborative International Dictionary of English v.0.48:

Retarded \Re*tard"ed\, n.
   having a limited or below normal mental ability; same as

   mentally retarded; -- used especially in relation to
      performance in academic tasks.
      [PJC]

WordNet (r) 3.0 (2006):

mentally retarded
    n 1: people collectively who are mentally retarded; "he started
         a school for the retarded" [syn: mentally retarded,
         retarded, developmentally challenged]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "mentally retarded":
   arrested, babbling, backward, blithering, burbling, crackbrained,
   cracked, crazy, cretinistic, cretinous, dithering, driveling,
   drooling, half-baked, half-witted, idiotic, imbecile, imbecilic,
   maundering, mentally defective, mentally deficient,
   mentally handicapped, mongoloid, moronic, not all there, retarded,
   simple, simpleminded, simpletonian, slobbering, subnormal