1520s, "deck with something becoming or ornamental, adorn, beautify," from Latin decoratus, past participle of decorare "to decorate, adorn, embellish, beautify," from decus (genitive decoris) "an ornament; grace, dignity, honor," from PIE root *dek- "to take, accept."
Earlier was decoren (early 15c.) with past-participle adjective decorat. Meaning "confer distinction upon by means of a badge or medal of honor" is from 1816. Related: Decorated; decorating.