act up
Britishverb
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informal (intr, adverb) to behave in a troublesome way
the engine began to act up when we were miles from anywhere
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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
“That’s not the idea. The idea is to have them clean their act up so that they do better and the stock goes up and their dividends go up,” Ryan said.
From Seattle Times • Feb. 26, 2024
“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
When I was dating the woman who became my wife, her 13-year-old would act up.
From Washington Post • Apr. 4, 2023
The doctor thought that was most likely what caused Haupt's heart to act up, but, other than high blood pressure, Dr. Otten thought his heart was all right.
From Nazi Saboteurs by Samantha Seiple
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.