The Collaborative International Dictionary of English v.0.48:

Retard \Re*tard"\, v. t. [imp. & p. p. Retarded; p. pr. & vb.
   n. Retarding.] [L. retardare, retardatum; pref. re- re- +
   tardare to make slow, to delay, fr. tardus slow: cf. F.
   retarder. See Tardy.]
   1. To keep delaying; to continue to hinder; to prevent from
      progress; to render more slow in progress; to impede; to
      hinder; as, to retard the march of an army; to retard the
      motion of a ship; -- opposed to accelerate.
      [1913 Webster]

   2. To put off; to postpone; as, to retard the attacks of old
      age; to retard a rupture between nations.
      [1913 Webster]

   Syn: To impede; hinder; obstruct; detain; delay;
        procrastinate; postpone; defer.
        [1913 Webster]

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):

retarded
    adj 1: relatively slow in mental or emotional or physical
           development; "providing a secure and sometimes happy life
           for the retarded" [ant: precocious]
    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:

75 Moby Thesaurus words for "retarded":
   arrested, babbling, back, backward, behind, behindhand, belated,
   blithering, blocked, bridled, burbling, checked, constrained,
   controlled, crackbrained, cracked, crazy, cretinistic, cretinous,
   curbed, delayed, delayed-action, detained, dithering, driveling,
   drooling, guarded, half-baked, half-witted, held up, hung up,
   idiotic, imbecile, imbecilic, impeded, in a bind, in abeyance,
   in check, in leading strings, in remission, inhibited, jammed,
   late, latish, maundering, mentally defective, mentally deficient,
   mentally handicapped, mentally retarded, mongoloid, moratory,
   moronic, never on time, not all there, obstructed, on leash,
   overdue, pent-up, restrained, set back, simple, simpleminded,
   simpletonian, slobbering, slow, slowed down, stopped, subnormal,
   tardy, under control, under discipline, under restraint,
   unpunctual, unready, untimely