mystery
some
unploughed
as-yet-unknown
enigma
inapplicability
unk-unk
submarine
some something or other
uncongeniality
inenubilable
dark
oojah
under the table
recondite
pseudosecret
occult
something or other
unintelligibility
semicryptic
something nasty in the woodshed
private
hidden in plain sight
deal
riddle wrapped up in an enigma
hinterland
undesirability
uncognoscible
unusual
hitch
noncategorizable
open secret
arcanum
inevident
enigmatical
heffalump
cabalistic
cryptical
stuff
sibylline
cryptic
kabbalistic
unascertained
qabalistic
underhanded
unrevealed
silent
supersecret
hinky
inconceivability