act up Definition in Cambridge British English Dictionary
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Definition of "act up" - British English Dictionary

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act up

phrasal verb with act uk   us   /ækt/ verb
If a person, especially a child, acts up, they behave badly: Sophie got bored and started acting up. If a machine or part of the body acts up, it does not perform as well as it should: My car always acts up in cold weather. Her shoulder was acting up (= hurting because of injury).
(Definition of act up from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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