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[for a thing or a person] to behave badly. This car is acting up again.
1. to behave badly Sometimes kids act up because they just want attention.
Usage notes: usually used to refer to children
2. to operate badly My computer began acting up and I lost a whole day's work.
3. to become active Her allergies acted up when she went hiking in the woods.
Usage notes: usually used to refer to a medical condition
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.