The Collaborative International Dictionary of English v.0.48:

Constrain \Con*strain"\, v. t. [imp. & p. p. Constrained; p.
   pr. & vb. n. Constraining.] [OF. constraindre, F.
   contrainde, L. constringere; con- + stringere to draw tight.
   See Strain, and. cf. Constrict, Constringe.]
   1. To secure by bonds; to chain; to bond or confine; to hold
      tightly; to constringe.
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            He binds in chains
            The drowsy prophet, and his limbs constrains.
                                                  --Dryden.
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            When winter frosts constrain the fields with cold.
                                                  --Dryden.
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   2. To bring into a narrow compass; to compress.
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            How the strait stays the slender waist constrain.
                                                  --Gay.
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   3. To hold back by force; to restrain; to repress.
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            My sire in caves constrains the winds. --Dryden.
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   4. To compel; to force; to necessitate; to oblige.
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            The love of Christ constraineth us.   --2. Cor. v.
                                                  14.
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            I was constrained to appeal unto C[ae]sar. --Acts
                                                  xxviii. 19.
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   5. To violate; to ravish. [Obs.] --Shak.
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   6. To produce in such a manner as to give an unnatural
      effect; as, a constrained voice.

   Syn: To compel; force; drive; impel; urge; press.
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The Collaborative International Dictionary of English v.0.48:

Constrained \Con*strained"\, a.
   Marked by constraint; not free; not voluntary; embarrassed;
   as, a constrained manner; a constrained tone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

constrained
    adj 1: lacking spontaneity; not natural; "a constrained smile";
           "forced heartiness"; "a strained smile" [syn:
           constrained, forced, strained]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "constrained":
   Olympian, aloof, arrested, backward, bashful, blank, bridled,
   chastened, chilled, chilly, cold, controlled, cool, curbed,
   detached, discreet, distant, expressionless, forbidding, frigid,
   frosty, guarded, hushed, icy, impassive, impersonal, in check,
   in control, in hand, in leading strings, in remission,
   inaccessible, incommunicable, inhibited, introverted, limited,
   modest, noncommittal, offish, on leash, pent-up, quelled, quiet,
   remote, removed, repressed, reserved, restrained, retarded,
   reticent, retiring, shrinking, slowed down, softened, stable,
   standoff, standoffish, subdued, suppressed, tempered, unaffable,
   unapproachable, uncongenial, undemonstrative, under control,
   under discipline, under restraint, unexpansive, ungenial,
   withdrawn