E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Jack of Dover.
A stockfish, hake salted and dried. The Latin for a hake is merlucius, and lucius is a jack or pike. Mer, of course, means the sea, and Dover, the chief Cinque Port, is used as a synonym. Also refuse wine collected into a bottle and sold for fresh wine. To do-over again. (See DOVER.)