“Oxford Dictionary”, Second Edition
Volume X page 245
“1709 Act 7 Anne c. 5...The Children of all natural-born subjects, born out of the Liegeance of her Majesty...shall be deemed...to be natural-born Subjects of this Kingdom.”
That 1709 law was I believe what the founders would have gone by. Anne was Queen of England in 1709.
There was this thing called the American Revolution where we rejected the rule of Kings and Queens.