Synonyms

act up

British

verb

informal (intr, adverb) to behave in a troublesome way

the engine began to act up when we were miles from anywhere

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

act up Idioms

Misbehave. For example, With an inexperienced rider, this horse always acts up . [c. 1900]
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.

“What makes a horse act up? Fear, mostly,” said Hollywood horse stunt coordinator Ryan Sturz.

From Los Angeles Times • Feb. 10, 2026

First, people who dress up don’t act up.

From Seattle Times • Jun. 19, 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 more serious the occasion, the more he'll act up.

From BBC • Oct. 24, 2022

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