E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Other Day (The).
The day before yesterday. The Old English other was used for second, as in Latin, unus, alter, tertius; or proximus, alter, tertius. Starting from to-day, and going backwards, yesterday was the proximus ab illo; the day before yesterday was the altera ab illo, or the other day; and the day preceding that was tertius ab illo, or three days ago. Used to express a short time ago.