From Longman Dictionary of Contemporary Englishcucumbercu‧cum‧ber /ˈkjuːkʌmbə $ -ər/ ●●○ noun [countable, uncountable] 
DFa long thin round vegetable with a dark green skin and a light green inside, usually eaten raw → cool as a cucumber at cool1(3)
DFa long thin round vegetable with a dark green skin and a light green inside, usually eaten raw → cool as a cucumber at cool1(3)Examples from the Corpus
cucumber• But Paula, dressed in light grey leggings, sweatshirt and a blue denim jacket, looks as cool as a cucumber.• Add noodles and cucumber to shrimp.• Chop the dill cucumber thinly and add to the bowl.• In fact, cucumbers are not outstanding in any known vitamin or mineral, either.• For me, cucumbers make a salad.• For rabbits, use red pimento for ears, strips of cucumber peel for whiskers and small pieces of olive for eyes.• Chop ¼ red pepper, 2 spring onions, 1 celery stalk, 2 cooked new potatoes, cucumber and 2 mushrooms.• Add water to barely cover the cucumbers.Origin cucumber (1300-1400) Old French cocombre, from Latin cucumis 