Be (as) cool as a cucumber

Be (as) cool as a cucumber

Meaning:

very calm and relaxed even in a difficult situation

If someone is (as) cool as a cucumber, he/she is very relaxed and/or calm when you expect him/her to be nervous or upset.

Example:

He had very bad news last night, but he came to the office as cool as a cucumber this morning as if nothing had happened.

She was cool as a cucumber even right after she was fired.