From Longman Dictionary of Contemporary Englishstraighten up phrasal verb1 STANDto make your back straight, or to stand up straight after bending down He remained bent over for several seconds before slowly straightening up.2 straighten something ↔ upTIDY to make something tidy3 American EnglishBEHAVE to begin to behave well after behaving badly You’d better straighten up, young lady! → straighten→ See Verb table
Examples from the Corpus
straighten up• If you're lifting something heavy, be careful not to hurt your back when you straighten up.• You'd better straighten up or you'll never make it out of high school.• She bent over the body, and when she straightened up there were tears in her eyes. 