arcane (adj.)

"hidden things, mysteries," 1590s, a direct adoption of the Latin plural of arcanum "a secret, a mystery," an important word in alchemy, from neuter of adjective arcanus "secret, hidden, private, concealed" (see arcane). It was occasionally mistaken for a singular and pluralized as arcanas, because arcana is far more common than arcanum.

Middle English arke, from Old English earc, Old Northumbrian arc, mainly meaning Noah's, but also the Ark of the Covenant (the coffer holding the tables of the law in the sanctum sanctorum), from Latin arca "large box, chest" (see arcane), the word used in the Vulgate. It also was borrowed in Old High German (arahha, Modern German Arche).

In general as "a coffer, a box" by late 12c. Also sometimes in Middle English "the breast or chest as the seat of emotions." From the Noachian sense comes the extended meaning "place of refuge" (17c.). As the name of a type of ship or boat, from late 15c. In 19c. U.S., especially a large, flat-bottomed river boat to move produce, livestock, etc. to market.