E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
When Dover and Calais meeti.e. never.
A jack of Dover. A jack is a small drinking vessel made of waxed leather, and a jack of Dover is a bottle of wine made up of fragments of opened bottles. It is customary to pour the refuse into a bottle, cork it up, and sell it as a fresh bottle. This is called dovering, a corruption of do-over, because the cork is done over with wax or resin.