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]

WordNet (r) 3.0 (2006):

unbent
    adj 1: not bent; "looking for an unbent nail"; "trees with
           straight unbent trunks make the best lumber"
    2: erect in posture; "sit straight"; "stood defiantly with
       unbowed back" [syn: straight, unbent, unbowed]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "unbent":
   arrowlike, dead straight, direct, even, flat, horizontal,
   in a line, level, lineal, linear, rectilineal, rectilinear, right,
   ruler-straight, smooth, straight, straight-cut, straight-front,
   straight-side, streamlined, true, unbending, unbowed, unbroken,
   uncurved, undeflected, undeviating, undistorted, uninterrupted,
   unswerving, unturned, upright, vertical