1650s, "authoritative utterance," from Late Latin dictationem (nominative dictatio) "a dictating, dictation," noun of action from past-participle stem of dictare "say often, prescribe," frequentative of dicere "to say, speak" (from PIE root *deik- "to show," also "pronounce solemnly"). Meaning "act or practice of expressing orally for another to write down" is by 1727.
