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"

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "unbend":
   abate, bate, bend, disentangle, ease, ease off, ease up, extend,
   flatten, give, give way, let down, let up, let up on, loose,
   loosen, loosen up, mitigate, put straight, rectify, relax, relent,
   remit, set straight, sit up, slack, slack off, slack up, slacken,
   slake, slow down, smooth, stand up, straighten, straighten out,
   straighten up, submit, unbrace, uncurl, unkink, unlax, unsnarl,
   unstrain, unstring, unwind, yield