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.
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            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.
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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]