c. 1200, ournement, "an accessory; something that serves primarily for use but also may serve as adornment; ornamental apparel, jewels," from Old French ornement "ornament, decoration," and directly from Latin ornamentum "apparatus, equipment, trappings; embellishment, decoration, trinket," from ornare "to equip, adorn," from stem of ordo "row, rank, series, arrangement" (see order (n.)).
The sense shift in English to "something employed simply to adorn or decorate, something added as an embellishment, whatever lends grace or beauty to that to which it is added or belongs" is by late 14c. (this also was a secondary sense in classical Latin). Meaning "outward appearance, mere display" is from 1590s. The figurative use is from 1550s; the meaning "one who adds luster to one's sphere or surroundings" is from 1570s.
"to adorn, deck, embellish," 1720, from ornament (n.). Middle English used ournen (late 14c.) in this sense, from Old French orner, from Latin ornare. Related: Ornamented; ornamenting.