primogenitor (n.)

"an ancestor, a forefather," 1650s, 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" (from PIE root *gene- "give birth, beget"). Related: Primogenital; primogenitary. The fem. form is primogenitrix (1875). The rights of a second son are secundogeniture.