'arch-enemy' originates from a combination of the prefix 'arch-' and the noun 'enemy'. The prefix 'arch-' ultimately comes from Greek 'arkhi-' (from 'arkhos') meaning 'chief' or 'principal' (via Latin/Old French 'archi-'), while 'enemy' comes from Old French 'enemi', from Latin 'inimicus' meaning 'not a friend'.

'arch-enemy' developed in English by combining 'arch-' with Middle English forms of 'enemy' (such as 'enemi'/'ennemi'); the compound appears in Early Modern English and later stabilized as 'arch-enemy' (with the variant one-word form 'archenemy').

Initially it meant 'chief enemy' in a literal sense; over time it has retained that core meaning but has also become widely used in figurative and literary contexts (e.g., a long-standing or defining adversary).