Arcanum

ar·ca·na
/ärˈkānə/
noun
noun: arcanum
secrets or mysteries.

ar·cane
/ärˈkān/
adjective
understood by few; mysterious or secret.

The word arcanum (pluralized as "arcana") came from Latin arcanus, meaning "secret," and entered English as the Dark Ages gave way to the Renaissance. It was often used in reference to the mysteries of the physical and spiritual worlds, subjects of heavy scrutiny and rethinking at the time.

Something arcane is understood or known by only a few people. The origin of arcane is Latin arcānus, "secret, closed," from arca, "a chest, box." Arcana (singular arcanum) are pieces of mysterious knowledge or information.

(literally just Arcane smut)