It's an English word. It originates in the French language (affaire ). It means subject, question, case, business. In Spanish we also give you the meaning of adventure, temporary affairs and is accepted as afer.
It is not Spanish but English, although in our language the original French affaire (aféer "theme, subject, situation") is used with the same meaning and without Castellanizing it. See English/affair ( compromised situation ), fato .