Originally Posted by Cam

Ditto, which at first glance seems a handy and insignificant sort of word, actually has a Roman past, for it comes from dictus, “having been said,” the past participle of the verb dcere, “to say.” In Italian dcere became dire and dictus became detto, or in the Tuscan dialect ditto. Italian detto or ditto meant what said does in English, as in the locution “the said story.” Thus the word could be used in certain constructions to mean “the same as what has been said” for example, having given the date December 22, one could use 26 detto or ditto for 26 December. The first recorded use of ditto in English occurs in such a construction in 1625. The sense “copy” is an English development, first recorded in 1818. Ditto has even become a trademark for a duplicating machine.