Why is the ninth month called September?
"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).
A system of inheritance in which land passes exclusively to the eldest son. Until the Industrial Revolution, this system severely restricted the freedom of younger sons, who were often forced into the military or the clergy to earn a living.