Illustrious

google

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mid 16th century: from Latin illustris ‘clear, bright’ + -ous.

wiktionary

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From Latin illūstris(“bright, shining; distinguished, prominent, illustrious”) +‎ -ous(“suffix forming adjectives from nouns, to denote possession or presence of a quality in any degree”). Illūstris is derived from illūstrō(“to brighten, illuminate; to make famous or illustrious”), from in-(“prefix meaning ‘in, inside’”) + lustrō(“to purify by making a sacrifice; to brighten, illuminate”) (from lustrō(“purificatory sacrifice”), possibly ultimately from Proto-Indo-European *lewk-(“bright; to shine”) or *lewh₃-(“to wash”)).

etymonline

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illustrious (adj.)

1560s, "distinguished by greatness, renowned," from Latin illustris "lighted, bright, brilliant;" figuratively "distinguished, famous," probably a back-formation from illustrare "make light, light up, illuminate," figuratively "embellish, distinguish, make famous" (see illustration). Replaced illustre in same sense (mid-15c.), from French illustre.