The Collaborative International Dictionary of English v.0.48:

Straighten \Straight"en\, v. t. [imp. & p. p. Straighted; p.
   pr. & vb. n. Straighting.]
   1. To make straight; to reduce from a crooked to a straight
      form.
      [1913 Webster]

   2. To make right or correct; to reduce to order; as, to
      straighten one's affairs; to straighten an account.
      [1913 Webster]

   To straighten one's face, to cease laughing or smiling,
      etc., and compose one's features.
      [1913 Webster]