Source : Webster's Revised Unabridged Dictionary (1913)

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}.

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

   Syn: To impede; hinder; obstruct; detain; delay;
        procrastinate; postpone; defer.

Source : WordNet®

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