1 A2 idiomatic informal
To behave badly or in a disruptive way, usually said of children.
"The kids started acting up as soon as the babysitter arrived."
inseparable
act up
B1 informal inseparable intransitive
To behave badly or in a difficult way, or (of a machine or body part) to stop working properly.
In plain English
When a person is being naughty or difficult, or when something stops working the way it should.
What does "act up" mean?
3 meanings, ordered from most common to least. Color-coded by CEFR level.
2 B1 idiomatic informal
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."
inseparable
3 B1 idiomatic informal
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."
inseparable
Literal vs figurative
Words literally mean
To perform ('act') in an upward (intensified) or wrong direction — misbehaving is like 'going against the expected flow'.
Actually means
When a person is being naughty or difficult, or when something stops working the way it should.
Usage tip
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.
Words that pair with "act up"
Natural word combinations native speakers use most often.
kids back knee computer car system
How to conjugate "act up"
The five tense forms you'll use most often.
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
Hear "act up" in the wild
Listen to native speakers using "act up" in real YouTube videos — click a clip to watch it on Looplines.
Other ways to say "act up"
Swap in when you want variety — tap a linked one to explore it.
Keep exploring
Jump to every phrasal verb built on the same verb, particle, or level.