“Cool as a cucumber”
It first appeared in English in the 18th century. The phrase was used in John Gay's poem “New Song on New Similes” in 1732, where he wrote, “I… cool as a cucumber could see the rest of womankind.”
Well, it turns out that cucumbers are actually cool to the touch, even on a hot day. This is because they have a high water content and can stay up to 6 or 7 degrees cooler than the outside temperature. So, when someone is “cool as a cucumber,” they're as calm and collected as a cucumber is cool and refreshing!
Examples:
- “Did you see how Jenny stayed cool as a cucumber during her big solo? She nailed it!”
- “I was so nervous about my job interview, but my husband was cool as a cucumber and helped me relax.”
- From online source, with editing
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Just checked this morning before posting: for most people, up to 60% of the human adult body is water!!!
No wonder some of us are cool as a cucumber, too. Hooray! 
