From Longman Dictionary of Contemporary Englishact up phrasal verb1 BEHAVEif children act up, they behave badly He’s a tough kid and he acts up a lot.2 MIBROKENif a machine or part of your body acts up, it does not work properly The computer is acting up again. → act→ See Verb table
Examples from the Corpus
act up• During his parents' divorce, Robert began acting up in class.Act-UpˈAct-Up an organization in the US and the UK that demands more help and support for people with AIDS. It is known for its strong active protests against companies or government departments if they treat people with AIDS unfairly. It is also known as the AIDS Coalition to Unleash Power. 