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.
        [1913 Webster]

WordNet (r) 3.0 (2006):

constrain
    v 1: hold back [syn: restrain, encumber, cumber,
         constrain]
    2: restrict; "Tighten the rules"; "stiffen the regulations"
       [syn: stiffen, tighten, tighten up, constrain]

Moby Thesaurus II by Grady Ward, 1.0:

169 Moby Thesaurus words for "constrain":
   abate, abridge, abstain, aggrieve, allay, alleviate, arrest,
   assuage, attemper, ban, bank the fire, bar, bastille, bear, bind,
   bit, blunt, bottle up, box up, bridle, cage, cause, cause to,
   chasten, check, cloister, coerce, compel, concuss, confine,
   contain, control, cool, cool off, coop, coop in, coop up, cork up,
   crib, crowd, crush, curb, curtail, damp, dampen, de-emphasize,
   deaden, decelerate, deprive, detain, dictate, diminish, disallow,
   dompt, downplay, drive, dull, encage, enclose, enforce, enjoin,
   extenuate, fence in, force, govern, grieve, guard, have, hem in,
   hinder, hold, hold at bay, hold back, hold down, hold fast,
   hold in, hold in check, hold in custody, hold in leash,
   hold in restraint, hold up, hurt, immure, impel, impound,
   incarcerate, inhibit, injure, insist upon, intern, jail, jam, jug,
   keep, keep back, keep from, keep in, keep in check,
   keep in custody, keep in detention, keep under control,
   keep within bounds, lay, lay under restraint, lenify, lessen,
   lighten, make, mew, mew up, mitigate, moderate, modulate,
   necessitate, oblige, obtund, pain, palliate, pen, pen up,
   play down, pound, prison, prohibit, pull, pull in, push, quod,
   rail in, reduce, reduce the temperature, refrain, rein, rein in,
   restrain, restrict, retard, retrench, seal up, set back, shackle,
   shotgun, shut in, shut up, slacken, slow down, smother, snub,
   sober, sober down, soften, squab, squash, squeeze, squish, stifle,
   straiten, subdue, suppress, tame, temper, tie, tone down,
   tune down, underplay, use force upon, wall in, weaken, withhold