Wiktionary
keep a cool head
vb. (context idiomatic English) to keep one's cool; to stay calm
Usage examples of "keep a cool head".
Normally Mildred would be able to keep a cool head, but after the niggling provocation of the past couple of days, it was to prove almost impossible.
I'll bet you have a special phrase to let that jacko know you're in danger, and he'd better keep a cool head and empty hands.