From Hull AWE
The abbreviation do (formerly written do) stands for ditto, which means "the same". Ditto, or its abbreviations, which include " " , is used to avoid writing out again something which one has just written.
- Etymological note: ditto was detto in its original Italian, which means 'the said ...' or 'aforesaid'. It was originally used, in its own language, only for the names of months, so that one could say "On the 2nd December, we ...; on the 19th ditto [i.e. December], we ... ". In England, where the form was influenced by the vowel of the Latin root dictus, the use of ditto spread to any repeated item in account books and similar commercial records. Tailors and dressmakers, describing garments made of the same cloth throughout, linings and so on, used to refer to a suit of dittoes. Ditto became a proprietary name (a trademark) for a type of duplicating machine in the early 20th century; but scholars of manuscripts had long used the term dittography for the error where a copyist has copied the same word or phrase twice (or even more) instead of once.