Yes, the inside of cucumbers are cooler than their surroundings, but that doesn’t mean as much as you think.
If you were to pick up a room temperature cucumber and insert a temperature probe into it, you’d find it to be somewhat cooler than the temperature of the room. You may even find it to be 10 to 20 degrees cooler on the inside. So, cucumbers are cool, relative to their surroundings. It has been suggested that the origin of idiom ‘cool as a cucumber’ is owed to this fact.
Perhaps even people living during the 1600’s had noticed this and invented the saying. As early as 1615 Francis Beaumont and John Fletcher used the expression in their play Cupid’s Revenge: “Young maids were as cold as cucumbers..”
Not Only Cucumbers are Cool
The problem is that cucumbers are not unique in their inner-coolness. A cucumber is mostly water containing up to 95%. Since water does not absorb heat as readily as the air, this water content means that it takes more energy to heat up the inside of the cucumber than it does the air that surrounds it. Thus, the cucumber will always be a little cooler than the outside. But here is the problem: The watermelon is also mostly water, as should be quite obvious. We don’t say “cool as a watermelon.”
In fact, cucumbers, watermelons, some other melons, and gourds are all related. We also don’t say “cool as a muskmelon.” However, the inside of any fruit that is mostly water will be cooler than the surrounding temperature. For this reason, these fruits have long been associated with cooling off during the summer.
But how “cool” is cool? If you were to check a cucumber in the field on a very hot summer’s day, it would still seem quite warm, not cool. And, growing up in the south, I’ve eaten enough warm watermelon to, personally, disbelieve the notion that watermelons are always cool. Remember, the hotter the air, the warmer the cucumber, watermelon, etc.
So, undoubtedly there is more to the origin of this idiom than the mere fact that cucumbers are cooler relative the surrounding air. Yes, the fact that the inside of a cucumber is around 20 degrees cooler than the air is may be a part of the story, but to understand the rest, all you have to do is say “cool as a watermelon” and compare it to “cool as a cucumber.” That’s right, the sounds have as much to do with the cucumber contribution to the idiom as the mere fact of their coolness. Many other water-laden fruits, after all, could have been used. And, notice that at some point we stopped saying “cold as a cucumber” and started saying “cool as a cucumber.”
This coincided with a shift in meaning. Instead of “cold as a cucumber” meaning emotionless, or even frigid, cool as a cucumber means calm and composed, especially under stress or during difficult situations.