The Collaborative International Dictionary of English v.0.48:

Restrain \Re*strain"\, v. t. [imp. & p. p. Restrained; p. pr.
   & vb. n. Restraining.] [OE. restreinen, F. restreindre, fr.
   L. restringere, restrictum; pref. re- re- + stringere to
   draw, bind, or press together. See Strain, v. t., and cf.
   Restrict.]
   1. To draw back again; to hold back from acting, proceeding,
      or advancing, either by physical or moral force, or by any
      interposing obstacle; to repress or suppress; to keep
      down; to curb.
      [1913 Webster]

            Restrain in me the cursed thoughts that nature
            Gives way to in repose!               --Shak.
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   2. To draw back toghtly, as a rein. [Obs.] --Shak.
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   3. To hinder from unlimited enjoiment; to abridge.
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            Though they two were committed, at least restrained
            of their liberty.                     --Clarendon.
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   4. To limit; to confine; to restrict. --Trench.
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            Not only a metaphysical or natural, but a moral,
            universality also is to be restrained by a part of
            the predicate.                        --I. Watts.
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   5. To withhold; to forbear.
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            Thou restrained prayer before God.    --Job. xv. 4.
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   Syn: To check; hinder; stop; withhold; repress; curb;
        suppress; coerce; restrict; limit; confine.
        [1913 Webster]

WordNet (r) 3.0 (2006):

restrained
    adj 1: cool and formal in manner [syn: restrained, reticent,
           unemotional]
    2: under restraint [ant: unrestrained]
    3: marked by avoidance of extravagance or extremes; "moderate in
       his demands"; "restrained in his response" [syn: moderate,
       restrained]
    4: not showy or obtrusive; "clothes in quiet good taste" [syn:
       quiet, restrained]
    5: prudent; "guarded optimism" [syn: guarded, restrained]

Moby Thesaurus II by Grady Ward, 1.0:

156 Moby Thesaurus words for "restrained":
   Attic, Ciceronian, Olympian, aesthetic, aloof, arrested, artistic,
   aseptic, backward, barred, bashful, beleaguered, beset, besieged,
   blank, blockaded, bound, bridled, cabined, caged, chaste,
   chastened, chilled, chilly, choice, classic, clear, cloistered,
   closed-in, cold, confined, constrained, controlled, cool, cooped,
   cordoned, cordoned off, corralled, cramped, cribbed, curbed,
   detached, direct, discreet, distant, easy, elegant, enclosed,
   excellent, expressionless, fenced, finished, forbidding, frigid,
   frosty, frugal, graceful, gracile, guarded, hedged, hemmed, hushed,
   icy, immured, impassive, impersonal, imprisoned, in check,
   in control, in good taste, in hand, in leading strings,
   in remission, inaccessible, incarcerated, incommunicable,
   inhibited, introverted, jailed, leaguered, limited, limpid, lucid,
   measured, mewed, moderate, modest, natural, neat, noncommittal,
   of choice, of quality, offish, on leash, paled, pellucid, penned,
   pent-up, perspicuous, plain, pleasing, polished, pure, quarantined,
   quelled, quiet, railed, reasonable, refined, remote, removed,
   repressed, reserved, retarded, reticent, retiring, round,
   shrinking, shut-in, simple, slowed down, sober, softened, sparing,
   stable, standoff, standoffish, stinting, straightforward, subdued,
   suppressed, tasteful, tasty, temperate, tempered, terse, trim,
   unaffable, unaffected, unapproachable, uncongenial,
   undemonstrative, under control, under discipline, under restraint,
   understated, unexcessive, unexpansive, unextreme, ungenial,
   unlabored, unobtrusive, walled, walled-in, well-chosen,
   withdrawn