Subject: Just be as cool as a cucumber.
A: Oh, my God. I have forgotten what I need to buy.
B: Do not worry. Just be as cool as a cucumber.
as cool as a cucumber
To be as cool as a cucumber的字面意思是“像黄瓜一样凉爽”， 比喻为“镇定，冷静，泰然自若”的意思。表示一个遇到麻烦时很冷静，很镇定，很放松。Cool在这里的意思不是“凉爽”，是“冷静的”的意思。使用时，表示对.....表现得冷静，在短语后面用上介词upon，即:be as cool as a cucumber uponsomething。
e.g. It's not everybody who can keep as cool as a cucumber in an emergency.
e.g. He is as cool as a cucumber upon every act of atrocity.
e.g. He kept as cool as a cucumber as the newspaper reporters shouted questions at him atonce.