Vladimir Nabokov may have intended to give inenubilable the opposite meaning to the Latin version. in the context he is referring to the mythical country of Zembla as being blue. if it were cloud-covered it would not be blue but gray. if it was cloudless, incapable of being made cloudy, it would be blue.
Origin of inenubilable
- From in- + Latin enubilare ‘make clear’ + -able.