The Collaborative International Dictionary of English v.0.48:

Ditto \Dit"to\, n.; pl. Dittos. [It., detto, ditto, fr. L.
   dictum. See Dictum.]
   The aforesaid thing; the same (as before). Often contracted
   to do., or to two "turned commas" ("), or small marks. Used
   in bills, books of account, tables of names, etc., to save
   repetition.
   [1913 Webster]

         A spacious table in the center, and a variety of
         smaller dittos in the corners.           --Dickens.
   [1913 Webster]