Originally: a member of the imperial family at Constantinople, reputedly born in a purple-hung or porphyry chamber. Later more generally: a child born after his or her father's accession to a throne. More generally: a member of an imperial or royal reigning family; one belonging to the highest or most privileged ranks.
Early 17th century; earliest use found in John Selden (1584–1654), lawyer and historical and linguistic scholar. From post-classical Latin porphyrogenitus (adjective) from Byzantine Greek πορϕυρογέννητος (also πορϕυρογεννήτης) from ancient Greek πορϕυρο- + γεννητός born (from γεννᾶν to beget, to bring forth, bear (from γέννα family) + -τός, suffix forming verbal adjectives); apparently so called either because born in a chamber called the Porphyra (from ancient Greek πορϕύρα purple), or because the emperors customarily wore purple.