Affair originated as the Old French phrase a faire ('to do'), from Latin ad facere, and entered English in the thirteenth century meaning 'business' before acquiring its romantic sense in the eighteenth.
An event or sequence of events of a specified kind; a matter of public interest or concern; a romantic or sexual relationship outside marriage.
From Old French afaire ('business, matter'), originally the phrase a faire ('to do'), from Latin ad ('to') and facere ('to do, make'). The word started as a description of anything that needed doing — business, tasks, concerns. Anglo-Norman compressed the phrase into a single word. English borrowed it in the thirteenth century meaning 'what one has to do' or 'business matters'. The plural 'affairs' (as in 'affairs of state') preserves this original breadth. The romantic sense
French gave English both 'affair' and 'ado' from similar origins. 'Ado' comes from Northern French a faire via a Scandinavian-influenced pronunciation. Meanwhile, the German borrowing Affäre skipped straight to the scandalous meaning, showing how the romantic sense spread across Europe in the eighteenth century.