E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Merry Dun of Dover.
A large mythical ship, which knocked down Calais steeple in passing through the Straits of Dover, and the pennant, at the same time, swept a flock of sheep off Dover cliffs into the sea. The masts were so lofty that a boy who ascended them would grow grey before he could reach deck again. (Scandinaoian mythology.)