Definition of “act up” - English Dictionary

“act up” in British English

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(Definition of “act up” from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)

“act up” in American English

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act up

phrasal verb with act us /ækt/ verb

(BEHAVE BADLY)

to behave badly:

The little girl was acting up, so her mother took her out of the restaurant.

act up

phrasal verb with act us /ækt/ verb

(OPERATE BADLY)

to operate badly:

My car is acting up again - I don't know what's wrong with it.

act up

phrasal verb with act us /ækt/ verb

(BECOME ACTIVE)

to become active:

Her allergies acted up when she went hiking in the woods.

(Definition of “act up” from the Cambridge Academic Content Dictionary © Cambridge University Press)