Strip away the centuries and 'affair' is simply 'something to do'. It entered English from Old French afaire, which was itself the compressed phrase a faire — 'to do' — built from Latin ad ('to') and facere ('to do, make'). In thirteenth-century English, an affair was any piece of business or matter requiring attention. The plural 'affairs' still carries this meaning: foreign affairs, business affairs, current affairs — all just things that need doing. The romantic sense arrived much later, in the eighteenth century, as a genteel euphemism for private business best not discussed openly. The same Latin root facere runs through dozens of English words — fact, factory, fashion, feasible, effect, perfect — all sharing that fundamental idea of doing or making. Affair stands out because it preserves not just the root but the entire Latin infinitive phrase, fossilised in a single English word.