'antimystic' is a Modern English compound formed from the prefix 'anti-' (from Greek 'antí', meaning 'against') + 'mystic' (from Greek 'mystikos', via Latin 'mysticus' and Middle English 'mystic').

'mystic' derives from Greek 'mystikos' (related to 'mystērion' and 'mystēs'), passed into Latin as 'mysticus' and into Middle English as 'mystic'; the prefix 'anti-' (from Greek 'antí') was later attached in Modern English to form 'antimystic'.

Originally elements meant 'secret/initiated' (mystikos) and 'against' (anti-); combined in Modern English to mean 'against mysticism' or 'opposed to mystical beliefs', a usage that reflects a straightforward compositional meaning rather than a large semantic shift.