The Collaborative International Dictionary of English v.0.48:

Unbend \Un*bend"\ ([u^]n*b[e^]nd"), v. t. [imp. & p. p. Unbent
   ([u^]n*b[e^]nt"); p. pr. & vb. n. Unbending.] [1st pref.
   un- + bend.]
   1. To free from flexure; to make, or allow to become,
      straight; to loosen; as, to unbend a bow.
      [1913 Webster]

   2. A remit from a strain or from exertion; to set at ease for
      a time; to relax; as, to unbend the mind from study or
      care.
      [1913 Webster]

            You do unbend your noble strength.    --Shak.
      [1913 Webster]

   3. (Naut.)
      (a) To unfasten, as sails, from the spars or stays to
          which they are attached for use.
      (b) To cast loose or untie, as a rope.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Unbend \Un*bend"\, v. i.
   1. To cease to be bent; to become straight or relaxed.
      [1913 Webster]

   2. To relax in exertion, attention, severity, or the like;
      hence, to indulge in mirth or amusement.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unbend
    v 1: straighten up or out; make straight [syn: straighten,
         unbend] [ant: bend, flex]
    2: unfasten, as a sail, from a spar or a stay
    3: free from flexure; "unbend a bow" [ant: bend, deform,
       flex, turn, twist]
    4: make less taut; "relax the tension on the rope" [syn:
       relax, unbend]
    5: become less tense, rest, or take one's ease; "He relaxed in
       the hot tub"; "Let's all relax after a hard day's work" [syn:
       relax, loosen up, unbend, unwind, decompress, slow
       down] [ant: tense, tense up]
    6: release from mental strain, tension, or formality; "unbend
       the mind from absorbing too much information"