Why is the ninth month called September?
mid-15c., "lawfully begotten," from Middle French legitimer and directly from Medieval Latin legitimatus, past participle of legitimare "make lawful, declare to be lawful," from Latin legitimus "lawful," originally "fixed by law, in line with the law," from lex (genitive legis) "law" (see legal). Transferred sense of "genuine, real" is attested from 1550s. Related: Legitimately.
1590s, from Medieval Latin legitimatus, past participle of legitimare (see legitimate (adj.)). Related: Legitimated; legitimating.