'antiposition' is built in modern English from the combining form 'anti-' (from Greek 'anti', meaning 'against' or 'opposite') plus the noun 'position' (from Latin 'positio', meaning 'a placing').

'position' came into English via Old French and Middle English from Latin 'positio' (from 'ponere', 'to place'); 'anti-' is from Greek 'anti' and was attached as a prefix in Modern English to form compounds such as 'antiposition'.

Initially it denoted a placing opposite or against something ('opposite placement'); over time the compound has retained that core sense, sometimes used more abstractly for contrast or counterpoint.