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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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
[for a thing or a person] to behave badly. This car is acting up again.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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."
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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.
The American Heritage® Dictionary of Phrasal Verbs. Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.