# act up

CEFR B2 · Phrasal verb

Source: https://englishreference.com/word/act-up
Updated: 2026-10-22

## Definition

to behave badly or cause trouble. You use this when a child is being difficult or when your phone starts doing weird things.

For teachers: to behave in a disruptive or unruly manner; alternatively, to malfunction or fail to operate normally. The verb is intransitive and does not take a direct object.

## Examples

- My old car is acting up again this morning.
- The children started to act up because they were bored during the long car journey.
- Despite the technician's recent repairs, the server began to act up during the peak traffic hours, causing significant delays.

## Pitfalls

- Not "My knee is acting up me." but "My knee is acting up." — The phrasal verb is intransitive and cannot take a direct object.

## Usage notes

- Used for both people behaving badly and machines malfunctioning.
- Teaching tip: Contrast with 'break down'; 'act up' implies it still works but poorly or intermittently, whereas 'break down' often means it has stopped completely.

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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.