And there is a word-
play with the French, or so I imagine with a reference not only
to "cloudless Zembla", but also the hint at "l'oubli" (unforgettable)? I
wonder if nubile and un-marriageable could be added since it always occurs
to me, although I think it doesn't realy apply ( inspite of our beloved
Charles Xavier's sexual inclinations).

So...did Beerbohm and VN have different definitions in mind? Or did some
misinterpret Beerbohm's meaning?

As often, Matthew, you have come up with another intriguing item. The Wiktionary gives:

 

meaning: incapable of being cleared of clouds; unclear, indistinct, inexplicable

1962:   Our blue inenubilable Zembla, and the red-capped Steinmann, and the motorboat in the seacave — Vladimir Nabokov, Pale Fire

1911:   For there is nothing in England to be matched with what lurks in the vapours of these meadows, and in the shadows of these spires - that inenubilable spirit, spirit of Oxford. - Max Beerbohm, Zuleika Dobson

 

However, the Beerbohm passage appears to be slightly misquoted. Another site gives:

“For there is nothing in England to be matched with
what lurks in the vapours of these meadows, and in the shadows of
these spires -- that mysterious, inenubilable spirit, spirit of Oxford.
Oxford! The very sight of the word printed, or sound of it spoken, is
fraught for me with most actual magic.

And on that moonlit night when I floated among the vapours of these
meadows, myself less than a vapour, I knew and loved Oxford as never
before, as never since.”

From this it would seem that Beerbohm is using the word correctly; ie it means clouded, mysterious, indistinct, inexplicable. By linking the word with "blue", however, VN, or Charles Kinbote rather, seems to be using it incorrectly, suggesting that it means "cloudless", as well as perhaps confusing it with Fr "inoubliable", ie, unforgettable.  No doubt this is a malapropism which should be charged to Kinbote, not VN.  

 

Charles