Lewis & Short

prīmōgĕnĭtus, a, um, adj. [primusgenitus], first-born (post-Aug. for maximus natu, etc.), Plin. 11, 40, 95, § 234 (al. primis genitis): vitulus, Pall. 1, 39: primogenita omnia deo offerenda, Sulp. Sev. Chron. 1, 18, 8: filius, Lact. 4, 11, 7; 3, 6, 2.
As subst.: prīmōgĕnĭta, ōrum, n., the right of the first-born, birthright, Vulg. Gen. 25, 33; Aug. Conf. 7, 9.