(as) cool as a cucumber
very calm or very calmly, especially when this is surprising:
She walked in as cool as a cucumber, as if nothing had happened.

[TahlilGaran] Advanced Learner's Dictionary ▲

cool as a cucumber
(as) cool as a cucumber
very calm and in control of your emotions.
When everything seems to be going wrong, she stays as cool as a cucumber.

[TahlilGaran] English Idioms Dictionary ▲

cool as a cucumber
to be very calm and brave, to be not worried or anxious
The woman was as cool as a cucumber when her canoe turned over in the river.

[TahlilGaran] English Idioms Dictionary ▲

cool as a cucumber
adj. phr., informal Very calm and brave; not nervous, worried, or anxious; not excited; composed.
Bill is a good football quarterback, always cool as a cucumber.

[TahlilGaran] English Idioms Dictionary ▲