act up

act up idiom
  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.

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