Nattily tells you how someone looks: neat, tidy, and a little fashionable, as if they chose their clothes with care. A man in a well-cut jacket and a clean hat might be “nattily dressed.” It is a slightly old, slightly British word — you will meet it in books and in older talk more than in a teenager’s mouth today. Young people are more likely to say smartly or stylishly. It is only about appearance, and only in a tidy, dapper way, not wild fashion. It does not mean “naturally,” and it is not about being born somewhere.