# act up

idiom

Source: https://englishreference.com/idiom/act-up
Updated: 2026-08-10

## Meaning

To misbehave; to cause trouble.

## Examples

- The children always act up when they're tired.
- My computer started to act up right before my important video call.
- The old car had behaved perfectly for years, but now it was beginning to act up at every traffic light.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.