"hidden things, secrets," 1590s, a primary adoption associated with Latin plural of arcanum "a key, a secret," from neuter of adjective arcanus "key, hidden, private, concealed" (see arcane). Occasionally recognised incorrectly as one and pluralized as arcanas because arcana is a lot more typical than arcanum.
(pl. ) of Arcanum
Besides the works already noticed, he wrote De arte critica (1597); De Antichristo (1605); Pro auctoritate ecclesiae in decidendis fidei controversiis libellus; Scaliger hypololymaeus (1607), a virulent attack on Scaliger; and latterly the anti-jesuitical works, Flagellum Jesuiticum (1632); Mysteria patrum jesuitorum (1633); and Arcana societatis Jesu (1635).