"an ancestor," late 15c., from Medieval Latin primogenitor, from Latin primo (adv.) "first in order of time; at first," from primus "first" (see prime (adj.)) + genitor "father," from genitus, past participle of gignere "to beget" (see genus). Related: Primogenital; primogenitary. The fem. form is primogenitrix (1875). The rights of a second son are secundogeniture.