arcane
ar-KANEadjMeaning known or understood by only a few; mysterious
Memory hook ARCA = CHEST: locked-away knowledge. The old wizard keeps his strangest spells locked in a chest, written in a script only he can read; they are arcane.
The old book was full of arcane rules for a card game no one plays anymore.
In context Only the oldest clerk understood the town’s arcane filing system, in which every folder was sorted by the color of its ink rather than by name or date.
Similar mysterious, obscure, little-known, specialized
Opposite common, familiar, well-known
From Latin arcanus, “secret, hidden,” from arca (chest, box).