early 15c., decoracioun, "the covering of blemishes with cosmetics;" 1580s, "action of adorning with something becoming or ornamental," from Medieval Latin decorationem (nominative decoratio), noun of action from past-participle stem of Latin decorare "to decorate, adorn, embellish, beautify," from decus (genitive decoris) "an ornament; grace, dignity, honor," from PIE root *dek- "to take, accept" (on the notion of "to add grace"),
Meaning "that which decorates" is from 1670s. As "a badge or medal worn as a mark of honor" (often in plural, decorations), also "the conferring of a badge or medal of honor," by 1816. In U.S., Decoration Day (by 1870) was another old name for Memorial Day (q.v.), when the graves of the Civil War dead from the North were decorated with flowers.
"one whose business is the decoration of dwellings or public edifices," 1700, agent noun in Latin form from decorate.