mid-15c., "lawfully begotten, born of parents legally married," from past participle of Old 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; legitimateness. The older adjective in English was legitime "lawful, of legitimate birth" (late 14c.), from Old French legitime, from Latin legitimus.
"establish the legitimacy of, make lawful," 1590s, from Medieval Latin legitimatus, past participle of legitimare "make lawful" (see legitimate (adj.)). Related: Legitimated; legitimating.