The Collaborative International Dictionary of English v.0.48:

Constraint \Con*straint"\, n. [OF. constrainte, F. constrainte.]
   The act of constraining, or the state of being constrained;
   that which compels to, or restrains from, action; compulsion;
   restraint; necessity.
   [1913 Webster]

         Long imprisonment and hard constraint.   --Spenser.
   [1913 Webster]

         Not by constraint, but by my choice, I came. --Dryden.

   Syn: Compulsion; violence; necessity; urgency.

   Usage: Constraint, Compulsion. Constraint implies strong
          binding force; as, the constraint of necessity; the
          constraint of fear. Compulsion implies the exertion of
          some urgent impelling force; as, driven by compulsion.
          The former prevents us from acting agreeably to our
          wishes; the latter forces us to act contrary to our
          will. Compulsion is always produced by some active
          agent; a constraint may be laid upon us by the forms
          of civil society, or by other outward circumstances.
          --Crabb.
          [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

169 Moby Thesaurus words for "constraint":
   abnegation, abstinence, aloofness, aplomb, arrest, arrestation,
   backwardness, bashfulness, blankness, caging, calm, calmness,
   check, chilliness, circumscription, coaction, coercion, coldness,
   composure, compulsion, compulsiveness, confinement, conservatism,
   continence, control, cool, cooling, cooling down, cooling off,
   coolness, cramp, curb, curtailment, deceleration, denial,
   detachment, discipline, discreetness, discretion, dispassion,
   distance, drive, driving, duress, enforcement, evenness, exigency,
   expressionlessness, forbearance, forcing, frigidity, frostiness,
   frugality, gentleness, goad, golden mean, guardedness,
   happy medium, hindrance, iciness, impartiality, impassiveness,
   impassivity, impelling, impersonality, impoundment, impulse,
   impulsion, inaccessibility, independence, inevitability,
   inhibition, injunction, interdict, introversion, irresistibility,
   judiciousness, juste-milieu, legal restraint, lenity, lockup,
   meden agan, middle way, mildness, moderateness, moderation,
   moderationism, modesty, monopoly, motive, necessity, neutrality,
   nonviolence, nothing in excess, obligation, obligement, pacifism,
   penning, pinch, possession, press, pressure, prohibition,
   protection, protectionism, protective tariff, prudence, push,
   rash impulse, rationing, rein, remoteness, renouncement,
   renunciation, repose, repression, reserve, reservedness, restraint,
   restraint of trade, restriction, retardation, reticence, reticency,
   retirement, retiring disposition, retrenchment, self-abnegation,
   self-command, self-conquest, self-control, self-denial,
   self-discipline, self-government, self-mastery, self-possession,
   self-restraint, serenity, slowing down, soberness, sobriety,
   sophrosyne, spring, spur, stability, standoffishness, steadiness,
   stress, subduedness, suppression, tariff wall, temperance,
   temperateness, thought control, tranquillity, unaffability,
   unapproachability, uncongeniality, undemonstrativeness,
   unexcessiveness, unexpansiveness, unextravagance, unextremeness,
   urge, urgency, via media, violence, withdrawal, withdrawnness