To honour the spookiness of October, today’s word is arcane.
The first known use of arcane was in 1547.
Arcane is an adjective; meaning known or understood by very few; mysterious; secret; obscure; esoteric.
Something arcane is understood or known by only a few people.
Related words are esoteric, impenetrable, cabalistic, mysterious, mystic, occult, recondite, unaccountable, unknowable.
Word origin: from Latin arcānus secret, hidden, from arcēre to shut up, keep safe.