cucumber:  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.
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.