To make something straight, to correct or improve a situation, or to stand or sit upright.
From 'straight' (Old English 'stræht,' past participle of 'stretch') plus '-en' suffix. Means to return something to a straight position or to make a situation orderly.
When we 'straighten out' a complicated situation, we're using the physical metaphor of untangling something bent, which is why physics and geometry use 'straighten' to mean 'make orderly and clear.'
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