Etymology
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arcanum (n.)
proper singular form of arcana (q.v.); in alchemy, a supposed great secret of nature.
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arcana (n.)
"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). Occasionally mistaken for a singular and pluralized as arcanas, because arcana is far more common than arcanum.
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