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.
      [1913 Webster]

            He binds in chains
            The drowsy prophet, and his limbs constrains.
                                                  --Dryden.
      [1913 Webster]

            When winter frosts constrain the fields with cold.
                                                  --Dryden.
      [1913 Webster]

   2. To bring into a narrow compass; to compress.
      [1913 Webster]

            How the strait stays the slender waist constrain.
                                                  --Gay.
      [1913 Webster]

   3. To hold back by force; to restrain; to repress.
      [1913 Webster]

            My sire in caves constrains the winds. --Dryden.
      [1913 Webster]

   4. To compel; to force; to necessitate; to oblige.
      [1913 Webster]

            The love of Christ constraineth us.   --2. Cor. v.
                                                  14.
      [1913 Webster]

            I was constrained to appeal unto C[ae]sar. --Acts
                                                  xxviii. 19.
      [1913 Webster]

   5. To violate; to ravish. [Obs.] --Shak.
      [1913 Webster]

   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):

constraining
    adj 1: restricting the scope or freedom of action [syn:
           confining, constraining, constrictive, limiting,
           restricting]