'arcanum' originates from Latin, specifically the word 'arcanum' (neuter of 'arcanus'), where 'arcanus' meant 'secret' (related to 'arca' meaning 'chest' or 'box').

'arcanum' passed from Classical/Medieval Latin into Late Latin/Medieval usage with meanings tied to secrecy and hidden things, and was borrowed into English (from Latin or via French/Medieval Latin) retaining its form 'arcanum'.

Initially it meant 'a hidden thing or secret' in Latin; over time it preserved that sense but also came to be used specifically for mystical, alchemical, or specialized remedies/knowledge in later usages.