What is Idiomatic Phrase 'Act Up'

The idiom "act up" is used to describe someone behaving badly or mischievously. It is believed to have originated in the late 19th century, when it was used to describe children who were misbehaving or acting out. It has since been used to refer to people of all ages who are acting in a disruptive or rebellious manner.

"Act up" is generally used as an admonition, warning someone that they are acting in an inappropriate or rebellious manner. It can also be used in a playful or affectionate way to describe someone who is being mischievous or acting out in a way that isn't really harmful. For example, one might say "Stop acting up, you're going to get yourself into trouble!" or "You're really acting up today, what mischief have you been getting into?"

It can also be used to describe a group of people who are being unruly or disruptive, such as "Everyone was acting up at the party last night."

Here are more examples using this idiom:

1. Stop acting up and pay attention to your studies. 
2. He's acting up, I'm sure he's up to something. 
3. The kids at the park were acting up and causing a ruckus. 
4. All the guests at the party were acting up and having a great time. 
5. I can't believe she's acting up again, she knows better than this. 
6. We need to get these kids to stop acting up and start behaving. 
7. She's been acting up lately, I don't know what's going on with her. 
8. It's not okay for you to keep acting up like this. 
9. He was acting up at school, so I had to come and pick him up. 
10. It's no use trying to reason with him when he's acting up.

"Act up" is not usually used to describe serious or dangerous behavior, and so it should not be used when someone is engaging in violent or criminal activities. 

Additionally, it shouldn't be used to describe someone with a mental health condition or disability that is causing them to behave in an unusual way. In such cases, it is best to use language that is more sensitive and understanding.