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act up — 2 meanings

One phrase, 2 meanings. Learn them one at a time, then test yourself.

phrasal verb2 meanings
act up
1misbehave2not work properly

Meaning 1 · most common

misbehave

  • informal

to behave badly or not follow the rules, especially for children or pets.

How oftenCommon
LevelB1
Used inMostly spoken

Examples

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.

Common mistake

Not: The kids are acting out again.

Say: 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.

More on “misbehave”Another example · Tell it apart · Goes with · Tip

Another example

When my little brother acts up in public, my parents usually try to calm him down quickly.

Tell it apart

The kids are acting up.

  • misbehaving

My old laptop's acting up.

  • playing up / not working right

Goes with

  • act up (misbehave)
Practice saying this
Next meaning: not work properly

Meaning 2

not work properly

  • informal

to stop working correctly or cause problems, especially for machines, devices, or body parts.

How oftenCommon
LevelB2
Used inMostly spoken

Examples

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.

Common mistake

Not: My back has been acting out lately.

Say: My back has been acting up lately.

It's 'act UP' (playing up); 'act out' means express feelings through bad behaviour — a different thing.

More on “not work properly”Another example · Tell it apart · Goes with · Tip

Another example

Sometimes, my internet connection acts up during important video calls, which can be very frustrating.

Tell it apart

My old car's acting up again.

  • not working properly

The kids are acting up again.

  • misbehaving

Goes with

  • act up (device/body part)
Practice saying this
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misbehavenot work properly