Opposer comes from 'oppose' (Old French 'opposer', influenced by Latin 'opponere', 'to set against') plus the agentive suffix '-er'. The suffix is part of the word's derivational shape, marking the final grammatical or semantic role while the base supplies the central meaning. As a noun, it carries the sense "one who opposes or resists something", with the older formation still helping explain the modern use. Relevant historical forms include Latin opponere and Latin ponere. Related words include oppose, opposition, opponent, opposite.