A billfold is a small, flat case, often leather, that you carry in a pocket. Inside it you keep paper money, credit cards, and sometimes an ID. It is the American name; in Britain people say wallet, and they will understand billfold but they themselves will not often use it. The “bill” here is the American word for a paper banknote — a dollar bill, a five-dollar bill — and the “fold” is the way the case closes, folding those notes in on themselves. Though actually, many billfolds today hold more cards than cash, but the old name has stuck. Think of the slim 皮夾 in a back pocket, not a larger bag.