1Of a newborn infant: that has reached full term; fully formed and living; (figurative) fully developed, full-fledged.
2Originally: of legitimate birth (implying noble lineage or descent). In later use chiefly: of unmixed ancestry, origin, or descent. Also occasionally as noun (with the and plural agreement): those who are of such ancestry, origin, or descent collectively. Now somewhat rare.
Old English; earliest use found in King Alfred (c848–899), king of the West Saxons and of the Anglo-Saxons. From full + born.