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]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "retard":
   arrest, backpedal, backwater, balk, block, bottle up, brake,
   bridle, check, clip the wings, confine, constrain, contain,
   control, cool, cool off, countercheck, curb, curtail, dam up, damp,
   dampen, decelerate, delay, detain, dompt, drag, draw rein, dunce,
   ease off, ease up, enjoin, fool, frustrate, govern, guard, hinder,
   hold, hold at bay, hold back, hold fast, hold in, hold in check,
   hold in leash, hold up, idiot, imbecile, impede, inhibit,
   intercept, interfere, interfere with, intermeddle, interrupt,
   intervene, jerk, keep, keep back, keep from, keep in,
   keep in check, keep under control, lag, lay under restraint,
   let down, let up, lose ground, lose momentum, lose speed,
   make late, meddle, moderate, moron, obstruct, oppose, prohibit,
   pull, pull in, reef, rein, rein in, relax, repress, resist,
   restrain, restrict, retrench, scotch, set back, slack off,
   slack up, slacken, slow, slow down, slow up, snub, stall, stay,
   stop, straiten, suppress, take in sail, throttle down, thwart,
   withhold