act up

Idioms & Phrases

act up

  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 © 1997. Published by Houghton Mifflin.
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