What does "cool as a cucumber" mean — and why is it funny?

informal

Meaning

Very calm and relaxed, especially in a stressful situation.

Where it comes from

In use since the 1700s. Cucumbers really do stay cooler inside than the air around them, which gave the comparison a grain of truth to start from.

Why it is funny

The fun is mostly in the sound — the soft repeated 'c' makes it pleasant to say — and in the oddness of holding up a vegetable as the model of composure. Of all calm things, a cucumber is a strange hero.

Used in a sentence

"The fire alarm went off and she stayed cool as a cucumber."