Wikipedia on Dueling: https://en.wikipedia.org/wiki/Duel [T]he office of 'second' developed into 'seconds' or 'friends' being chosen by the aggrieved parties to conduct their honour dispute. These friends would attempt to resolve a dispute upon terms acceptable to both parties and, should this fail, they would arrange and oversee the mechanics of the encounter. Here, "second" refers to a specific role, much like saying "his barber" or "his baker". So the "second" is a chosen person.