Definition of cucumber noun from the Oxford Advanced Learner's Dictionary

cucumber

noun
ˈkjuːkʌmbə(r)
 
; ˈkjuːkʌmbər
 
[countable, uncountable]
 
a long vegetable with dark green skin and light green flesh, that is usually eaten raw
see also sea cucumber
Idioms
see
(as) cool as a cucumber at cool adjective