the.com/affair

means A romantic or sexual relationship, typically secret and outside a committed partnership — or, more broadly, any matter, event, or thing being handled ('the whole affair was a mess').

from From French à faire, literally 'to do' — the same humble phrase that gives us the sense of a 'thing to be done' or a matter of business. English borrowed it in the Middle Ages simply meaning 'matters' or 'doings.' The romantic sense crept in later, from phrases like 'affair of the heart' — a 'thing to attend to' that happened to involve someone else's spouse. So the word's whole scandalous reputation grew out of pure bureaucratic vocabulary: before it meant forbidden love, it just meant paperwork.