act up

verb

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

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

Idioms and Phrases

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

"When people hear leaders using that language it allows the average Joe Bloggs to act up and that's why we had the race riots of Summer 2024," she added.

From BBC

RCA still sees plenty of room for Sleep Token and its ilk to win freshly curious audiences, and conjure even more metal acts up the charts.

From Los Angeles Times

I started acting up simply to keep myself occupied.

From Los Angeles Times

In the early years, he said, he acted up, raking up disciplinary infractions that landed him in solitary confinement.

From Los Angeles Times

I told her the camera is going to be constantly running, so when she’s acting up — and I wanted to open the film with that — she knew that was being recorded.

From Salon