c. 1600, "separate, separated from others that are similar or contiguous" (a sense now obsolete), past-participle adjective from distinguish. Sense of "better known than others in the same class, separated from the generality by superior abilities, character or achievement," hence "famous, celebrated," is by 1714; meaning "having an air of distinction" is from 1748.
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.