'oppositionist' originates from Modern English, formed from the noun 'opposition' + the agent suffix '-ist', where 'opposition' derives from Latin 'oppositio' and the elements of that Latin root relate to placing against.

'oppositionist' developed in English by adding the suffix '-ist' (via Latin/Greek '-ista/-istēs') to the noun 'opposition'. The noun 'opposition' came into English from Old French/Medieval Latin forms of Latin 'oppositio', which ultimately traces to Latin 'opponere' (ob- 'against' + ponere 'to put').

Initially the root words referred to the act or condition of placing against or opposing; over time the compound 'oppositionist' came to mean a person who opposes (general sense) and then also a political opponent (specific sense).