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1590s, "to practice dictation, say aloud for another to write down," from Latin dictatus, past participle of dictare "say often, prescribe," frequentative of dicere "tell, say" (see diction). Sense of "to command" is 1620s. Related: Dictated; dictates; dictating.
1590s, from Latin dictatum "something dictated," noun use of neuter past participle of dictare (see dictate (v.)).