extremely calm; imperturbable. (*Also: as ~.) Joan felt nervous, but she acted as cool as a cucumber.The politician kept cool as a cucumber throughout the interview with the aggressive journalist.
Calm and composed, self-possessed, as in Despite the mishap Margaret was cool as a cucumber. This idiom may be based on the fact that in hot weather the inside of cucumbers remains cooler than the air. [c. 1600] For a synonym, see cool, calm, and collected.