Brewer's: Born in the Purple
(a translation of porphyrogenitus ). The infant of royal parents in opposition to born in the gutter, or child of beggars. This has nothing to do with the purple robes of royalty. It refers to the chamber lined with porphyry by one of the Byzantine empresses for her accouchement. (See Nineteenth Century, March, 1894, p. 510.)
“Zoe, the fourth wife of Leo VI., gave birth to the future Emperor Constantine Porphyrogenitus in the purple chamber of the imperial palace.” —Finlay: History of the Byzantine and Greek Empires, vol. i.