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ar·cane

ärˈkān/

adjective

adjective: arcane

1. understood by few; mysterious or secret.

   "modern math and its arcane notation"

   synonyms:

   mysterious, secret; More enigmatic, esoteric, cryptic, obscure, abstruse, recondite, recherché, impenetrable, opaque "processes as old and arcane as the language of the law"

Origin

mid 16th century: from Latin arcanus, from arcere ‘to shut up,’ from arca ‘chest.’