'antithetic' originates from Greek, specifically the word 'antithetikos', where 'anti-' meant 'against' and 'tithenai' (from which the participial form derives) meant 'to place'.

'antithetic' changed from the Greek 'antithetikos' into Late Latin/Neo-Latin forms such as 'antitheticus' and entered English (via scholarly and ecclesiastical Latin and later borrowings) to become the modern English 'antithetic'.

Initially, it meant 'placed against' or 'set in opposition'; over time it evolved to the current usage meaning 'directly opposed, contrasting, or constituting an antithesis'.