To retain one's composure; stay calm; cool it: We'd all better keep our cool and not provoke him(1950s+ Beat & cool talk)
Idioms and Phrases with keep one's cool
keep one's cool
Retain one's composure and poise, as in Billy keeps his cool, no matter what the situation. This slangy usage dates from the mid-1900s, as do the antonyms blow one's cool and lose one's cool, as in Try not to blow your cool in front of the team, or Dad lost his cool when he saw Jim playing with matches. Also see keep cool