EDNote: For the record, here's what the OED gives
(~SB): That cannot be cleared of clouds or mist, or (fig.) of obscurity; indistinct; inexplicable.
1903 Sat. Rev. 7 Feb.
169/1 This business of the avalanche is treated by the critics as
something quite inenubilable. 1911 BEERBOHM Zuleika D. xii. 191 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.
JM: So it seems that the word "inenubilable" was first coined in 1903
and reappropriated by Beerbohm in 1911.
Kinbote was next.
If we consider its first, second and third uses as metaphors that
made it into the OED, that "inenubilable" must originally
have meant just that, an "uncloudable" invention, or whatever else
different readers may associate to it, with no particular "original truth".