If someone is nonchalant, they are very calm and relaxed, and they don't seem to care much about anything.
Being nonchalant can make people think someone is cool or doesn't worry about things.
Examples:
She was so nonchalant about the test, like she didn't even study.
He walked into the room with a nonchalant attitude.
The nonchalant boy didn't seem to care about anything.
The cat was nonchalant about the dog barking.
She was so nonchalant, she didn't even notice the big mess.
He was nonchalant when he found out he won the big prize.