The term is a combination of "city" in the sense of "an incorporated administrative district", and "proper" in the sense of "strictly limited to a specified thing, place, or idea" or "strictly accurate".
英文的“city proper”是由行政区划意义上的“city”（市，an incorporated administrative district）和表示“某事物、的严格界定”（strictly limited to a specified thing, place, or idea）或“严格准确”（strictly accurate）的“proper”组合而成。