act up
Britishverb
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Misbehave. For example, With an inexperienced rider, this horse always acts up . [c. 1900]
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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.”
Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
I felt like all these isolated incidents were speaking to me to act up.
From BBC ● Mar. 7, 2026
“With ‘Dead Man Walking,’ Sean was the first professional I saw act up close, the person I think I learned the most about acting from.
From Los Angeles Times ● Dec. 20, 2023
He said he did everything to prepare for St. Andrews, only for his leg to act up on him and lead to an early departure packed with emotion.
From Seattle Times ● Nov. 29, 2022
Lopez says she hopes to keep playing for many years, even if her knee is starting to act up a bit.
From Reuters ● Aug. 5, 2022
But my son, too tired and too excited, began to act up.
From "Newjack: Guarding Sing Sing" by Ted Conover
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
