'antithet' originates from Ancient Greek, specifically from the verb 'antitithenai' (ἀντιτίθημι), where the prefix 'anti-' meant 'against' and 'tithenai' meant 'to place.'

'antithet' was formed via Late Latin/Medieval Latin borrowings of Greek elements and appeared in Middle English as a rare or scholarly noun derived from those classical roots, aligning with words like 'antithesis' and 'antithetical.'

Initially, it meant 'one who is placed against or who opposes,' and over time the sense has remained largely the same but the word itself became rare or obsolete in modern usage, supplanted by terms like 'opponent' or 'antithesis.'