"right of succession of the first-born," c.1600, from French primogeniture and directly from Medieval Latin primogenitura, from Late Latin primogenitus "first-born," from Latin primo (adv.) "first in order of time," from primus "first" (see prime, adj.) + genitus, past participle of gignere "to beget" (see genus). Earlier it meant simply "fact of being first-born" (1590s).