What is the meaning of idiom not upset

The idiom "not upset" doesn't really exist as a stand-alone idiom. It's a simple, literal phrase meaning that someone is not experiencing any emotional distress or anger.

It's often used in contrast to being upset, for example:

* "Despite the loss, he seemed not upset at all."

* "I was worried she would be upset, but she was not upset by the news."

It's important to note that while "not upset" is grammatically correct, it might not be the most descriptive or nuanced way to express someone's emotional state. There are many other idioms and phrases you could use depending on the specific context, such as:

* Calm as a cucumber

* Unfazed

* Taking it in stride

* Not bothered

* Perfectly fine

* Unaffected

Choosing the right phrase depends on the specific situation and the intended meaning.