English Reference

Phrasal verb

act up

Definition

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

phrasal verb. 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.”

Examples

simple

“My old car is acting up again this morning.”

contextual

“The children started to act up because they were bored during the long car journey.”

complex

“Despite the technician's recent repairs, the server began to act up during the peak traffic hours, causing significant delays.”

Usage

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.

Pitfall

My knee is acting up me.

My knee is acting up.

The phrasal verb is intransitive and cannot take a direct object.

Phrasal verb

Particles

up

Separability

inseparable

Pattern

act + up

Synonyms

misbehavemalfunctionplay upcause trouble

Antonyms

behavework perfectly

Grammar

Phrasal verbs (introduction)

This entry

Exam level
CambridgeCambridgeCambridge English QualificationsThe exam suite from Cambridge University Press & Assessment, from A1 Movers up to C2 Proficiency. The benchmark names double as CEFR labels.B2 FirstIELTSIELTSInternational English Language Testing SystemThe 0–9 band test co-owned by the British Council, IDP, and Cambridge. Used for university entry, work, and migration.5.5–6.5TOEFL iBTTOEFL iBTTest of English as a Foreign Language, Internet-Based TestETS's academic English test, the standard route for US university admissions. Scored 0–120, with a 1–6 scale since 2026.1–6 scale: 4.0–4.572–94TOEICTOEICTest of English for International CommunicationETS's workplace English test, big in Japan and Korea. Listening & Reading scores 10–990 and tops out at C1.785–944
Level
B2 · Upper Intermediate
Updated

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English Reference