Cucumbers taste great in a salad and they also work wonders on tired eyes, especially after a sleepless night. Cucumbers are cool to touch. This is because, the inside of cucumbers are 20% cooler than the outside air.
And that explains the phrase, cool as a cucumber which means to remain calm and relaxed and in full control of your emotions. The word cool in the phrase means calm, rather than having a cool temperature. The phrase can be used in a number of situations.
For example: The politician remained cool as a cucumber , even when provoked by the interviewer.
I wish I could remain cool as a cucumber even when I face a huge audience. Usually, I get so nervous and mix up my words.