act up

B1 informal inseparable intransitive

In simple words

When a person is being naughty or difficult, or when something stops working the way it should.

Literal meaning: To perform ('act') in an upward (intensified) or wrong direction — misbehaving is like 'going against the expected flow'.

Meanings

1 A2 idiomatic informal

very common

To behave badly or in a disruptive way, usually said of children.

"The kids started acting up as soon as the babysitter arrived."

Grammar: inseparable

2 B1 idiomatic informal

very common

Of a machine, system, or body part: to stop functioning correctly or to cause problems.

"My old laptop has been acting up again — it keeps crashing."

Grammar: inseparable

3 B1 idiomatic informal

common

Of an injury or health condition: to cause pain or discomfort again after a period of improvement.

"Her knee has been acting up since she went back to training."

Grammar: inseparable

Usage notes

Very common in everyday informal English. 'Play up' is the British English equivalent. Used for children misbehaving, machines malfunctioning, or injuries recurring. Almost always intransitive.

Commonly used with

kids back knee computer car system

Forms

Base

act up

I/you/we/they

3rd person

acts up

he/she/it

Past simple

acted up

yesterday

Past participle

acted up

have + pp

-ing form

acting up

continuous

Understand "act up" better

Try:

Real video examples

Video examples are being collected. Check back soon.

Synonyms

misbehave play up malfunction cause trouble play up go wrong

Want to master this phrasal verb?

Practice "act up" on Looplines