Legitimate
late Middle English (in the sense ‘born of parents lawfully married to each other’): from medieval Latin legitimatus ‘made legal’, from the verb legitimare, from Latin legitimus ‘lawful’, from lex, leg- ‘law’.
wiktionary
From Middle English legitimat, legytymat, from Medieval Latin lēgitimatus, perfect passive participle of Latin lēgitimō(“make legal”), from Latin lēgitimus(“lawful”), originally "fixed by law, in line with the law," from Latin lēx(“law”). Originally "lawfully begotten".
The verb was derived from the adjective by conversion.
etymonline
legitimate (adj.)
mid-15c., "lawfully begotten, born of parents legally married," from past partixiple 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.
legitimate (v.)
"establish the legitimacy of, make lawful," 1590s, from Medieval Latin legitimatus, past participle of legitimare "make lawful" (see legitimate (adj.)). Related: Legitimated; legitimating.
