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1. Of a person, to behave poorly or inappropriately. If you act up in class, you will be sent to the principal's office.
2. Of a thing, to malfunction or operate incorrectly. My car's transmission started acting up during my commute to work. My phone acted up again this morning; I think I need to take it to a professional.
3. Of a medical condition, to become problematic or troublesome, usually after a period of remission. Jake played football with his friends today, and now his old knee injury is acting up. It's springtime, so of course my allergies are acting up again.
[for a thing or a person] to behave badly. This car is acting up again.
1. Misbehave. For example, With an inexperienced rider, this horse always acts up. [c. 1900]
2. Malfunction, as in I'm not sure what's wrong with my car, but the transmission is acting up. In both usages up means "abnormally."
1. To misbehave: The driver stopped the school bus because the kids were acting up.
2. To cause problems by operating strangely or unexpectedly: The thermostat suddenly started acting up, and now it's always too hot or too cold in here.
3. To become active or troublesome after a period of operating normally: My knee starts acting up when the weather is cold.