'Cool' by itself as an adjective has several different meanings. If you hear someone say "He's cool." when referring to someone, it requires some thought. What meaning is intended? Is he cool, as in not warm? Is he cool, as in not friendly and sociable? Or, is he cool, as in uniquely interesting in ways you admire. One must consider the context of the conversation, and the semantics of the preceding sentences.
'Cool as a cucumber' is an idiom that is used in reference to a person and their personality. Consider the characteristics of the vegetable: a cucumber is full of water, and its temperature is cool. It is refreshing even when eaten while at a picnic on a hot summer day. They hang in the shade of the vines on which they grow, and are relatively cool in temperature. There are many varieties of cucumbers native to India, where it is very hot. Yet, the cucumber maintains its relatively cool temperature. That's the interesting part.
We use this idiom when we make a direct reference to a person who remains very calm and composed in difficult circumstances. In can be said that when a situation is 'heated,' a person can turn red with anger and reach his or her 'boiling point.' People often say things like "Oooh, that kid...sometimes he makes my blood boil!" Well, this means that the persons feelings are beyond control. Remaining calm in spite of difficult circumstances is difficult...yet for some people, it is natural. Some people just don't get upset. (At least they don't show it!)
Example scenarios:
Parents talking at their kids' soccer game:
Abdul's Mom: Oh, that referee makes me so mad. He's calling all the penalties against our team. He's thrown the red flag on Abdul twice this game already, and it's not even half time!
Sam's Dad: Don't worry about it. Even the refs here are just learning. Give them a chance...the kids are at least learning all the rules.
Abdul's Mom: I don't know how you do it! When Sam was tripped by that other kid and the ref did nothing, you stood there as cool as a cucumber. I would have lost it! I probably would have yelled at the ref!
Sam's Dad: Well, I don't want to embarrass Sam, and I don't want to embarrass myself. Afterall, it's just a game.
Friends talking:
"Have you seen that new girl? She got her locker next to all those weirdos by the exit...you know, the punk rockers or whatever they are...the ones with the shaved heads and piercings and tattoos and all that. Anyway, they were kind of hassling her about wearing her work uniform to school, but she didn't even react. She just smiled and acted as cool as a cucumber."
In the workplace office:
Don: That stupid photocopier really pisses me off! I got ink all over my hands! The boss keeps getting the same ink cartridges refilled...maybe five or six times already, and they are somehow leaking toner.
Adrian: Hahahaha! That's hilarious! Hey, I do the same thing at home. Those cartidges can be used at least a half a dozen times. Maybe you need to read the manual or something! Ahahaha.
Sandra: I agree...I got ink on my hands the other day too, and I had to wash my hands several times before it completely came off. Mark got it all over his suit last fall. It didn't really bother him much at all. He acted as cool as a cucumber. I don't get how does he does it. Nothing seems to rattle him. He just sent an email to the boss saying that he's gone home for the day, and left. No-one said anything about it. I heard that the boss even gave him a gift card at Eddy's Mens Wear.
Don: Yeah, Mark is pretty relaxed about things.
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Definition of idiom:
An idiom is a phrase or expression that typically presents a figurative, non-literal meaning attached to the phrase. An idiom's figurative meaning is different from the literal meaning of the component words. Idioms are considered formulaic language with figurative meaning.