The Framers didn’t define a single word or phrase in the Constitution, but that doesn’t mean we can’t discover what their original intent was.
We need only consult their letters and reference works of the day. Doing so reveals what the common understanding of the terms were at that time.
All persons born in the allegiance of the king are natural-born subjects, and all persons born in the allegiance of the United States are natural-born citizens. Birth and allegiance go together. Such is the rule of the common law, and it is the common law of this country, as well as of England.
Justice Swayne, United States v. Rhodes, 1 Abbott, US 28 (Cir. Ct. Ky 1866)