As for a "legal thing," the Framers used many words and phrases: year, dollar, natural-born are examples. They used them because they were words that everyone of their time understood well. The phrase natural-born was so well understood that neither Story nor Rawles commented upon the natural-born citizen requirement in their commentaries. But Rawles does use the phrase elsewhere in his commentary in a way that makes it obvious what his understanding was.
ML/NJ
Did everyone have birth certificates then?