cucumberyoudaoicibaDictYouDict[cucumber 词源字典]
cucumber: [14] English acquired this word as cucumer, by direct borrowing from Latin cucumer, which may originally have been a word of some pre-Italic Mediterranean language. The form spelled with a b did not appear until the 15th century. It seems to have been a blend of Middle English cucumer and Old French coucombre, which itself ultimately derived from Latin cucumer. Spellings based on the Old French form led to a pronunciation of the first syllable as ‘cow’, which persisted until the early 19th century.
[cucumber etymology, cucumber origin, 英语词源]
cucumber (n.)youdaoicibaDictYouDict
late 14c., 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 common form 17c.-18c., and that pronunciation lingered into 19c. 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.