to behave badly or not follow the rules, especially for children or pets.
The kids started to act up as soon as the teacher left the room.
A: Why is your dog barking so much? B: Sorry, he's just acting up because he's hungry.
When my little brother acts up in public, my parents usually try to calm him down quickly.
Common mistake
✗The kids are acting out again.
✓The kids are acting up again.
For kids or pets misbehaving, use 'act up' (BrE 'play up'); 'act out' means expressing feelings through bad behaviour.
Tell it apart
The kids are acting up.
misbehaving
My old laptop's acting up.
playing up / not working right
Use this when talking about kids or animals not behaving well. For adults, it's usually a joke.Don't confuse with 'act out', which means to express feelings through actions.
to stop working correctly or cause problems, especially for machines, devices, or body parts.
My laptop started acting up right before my online meeting.
A: Why are you limping? B: My knee's acting up again. I think I need to rest it.
Sometimes, my internet connection acts up during important video calls, which can be very frustrating.
Common mistake
✗My back has been acting out lately.
✓My back has been acting up lately.
It's 'act UP' (playing up); 'act out' means express feelings through bad behaviour — a different thing.
Tell it apart
My old car's acting up again.
not working properly
The kids are acting up again.
misbehaving
Use this when something like a phone, computer, or even your knee is not working as it should.Don't confuse with 'break down', which usually means something stops working completely.