as cool as a cucumber，直接翻译是“像黄瓜一样凉”，实际含义是(尤指意料之外地)非常冷静，泰然自若。

【例句】
Hearing the news, everyone in the room panicked except for Tracey-she looked as cool as a cucumber.
听到那条消息后，房间里的所有人都陷入了慌乱之中，除了特蕾西，她看起来泰然自若。

Though it was his first great venture, he was now as cool as a cucumber.
虽然这是他第一次的大冒险事业，但是他却非常冷静。