From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Straighten \Straight"en\, v. t.
A variant of Straiten. [Obs. or R.]
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
straighten
v 1: straighten up or out; make straight [syn: straighten,
unbend] [ant: bend, flex]
2: make straight [syn: straighten, straighten out]
3: get up from a sitting or slouching position; "The students
straightened when the teacher entered"
4: put (things or places) in order; "Tidy up your room!" [syn:
tidy, tidy up, clean up, neaten, straighten,
straighten out, square away]
5: straighten by unrolling; "roll out the big map" [syn: {roll
out}, straighten]
6: make straight or straighter; "Straighten this post";
"straighten hair"