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:

Retard \Re*tard"\, v. i.
   To stay back. [Obs.] --Sir. T. Browne.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Retard \Re*tard"\, n.
   1. Retardation; delay.
      [1913 Webster]

   2. A mentally retarded person. [Colloq. and disparaging]
      [PJC]

   3. a person who is stupid or inept, especially in social
      situations. [Colloq. and disparaging]
      [PJC]

   Retard of the tide, or Age of the tide, the interval
      between the transit of the moon at which a tide originates
      and the appearance of the tide itself. It is found, in
      general, that any particular tide is not principally due
      to the moon's transit immediately proceeding, but to a
      transit which has occured some time before, and which is
      said to correspond to it. The retard of the tide is thus
      distinguished from the lunitidal interval. See under
      Retardation. --Ham. Nav. Encyc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

retard
    n 1: a person of subnormal intelligence [syn: idiot,
         imbecile, cretin, moron, changeling, half-wit,
         retard]
    v 1: cause to move more slowly or operate at a slower rate;
         "This drug will retard your heart rate"
    2: be delayed
    3: slow the growth or development of; "The brain damage will
       retard the child's language development" [syn: check,
       retard, delay]
    4: lose velocity; move more slowly; "The car decelerated" [syn:
       decelerate, slow, slow down, slow up, retard] [ant:
       accelerate, quicken, speed, speed up]