Definition of act up phrasal verb from the Oxford Advanced Learner's Dictionary to behave badly The kids started acting up. The Oxford Learner’s Thesaurus explains the difference between groups of similar words. Try it for free as part of the Oxford Advanced Learner’s Dictionary app

act up (third-person singular simple present acts up, present participle acting up, simple past and past participle acted up) (intransitive, idiomatic) To misbehave; to cause trouble. My allergies act up during hay fever season. Children may act up in class in an effort to get attention.

to activate or start (especially describing illnesses or problems). to behave badly: — My daughter always acts up at the supermarket when I refuse to buy her candy. — If you don't stop acting up I'm going to send you down to the principal's office.

Act up refers to a form of expressive behavior that challenges societal norms, often used as a means of political activism through body art and performance.