"common running garden plant," cultivated from earliest times in many Old World countries, also the long, fleshy fruit of the plant, late 14c., cucomer, from Old French cocombre (13c., Modern French concombre), from Latin cucumerem (nominative cucumis), perhaps from a pre-Italic Mediterranean language. The Latin word also is the source of Italian cocomero, Spanish cohombro, Portuguese cogombro. Replaced Old English eorþæppla (plural), literally "earth-apples."
Cowcumber was the common form of the word in 17c.-18c., in good literary use and representing the modern evolution of the Middle English form. Cucumber is an attempted reversion to Latin. In 1790s the pronunciation "cowcumber" was standard except in western England dialects and "coocumber" was considered pedantic, but 30 years later, with the spread of literacy and education "cowcumber" was limited to the ignorant and old-fashioned.
It was planted as a garden vegetable by 1609 by Jamestown colonists. Phrase cool as a cucumber (c. 1732) embodies ancient folk knowledge confirmed by science in 1970: inside of a field cucumber on a warm day is 20 degrees cooler than the air temperature.