E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Wave.
The ninth wave. A notion prevails that the waves keep increasing in regular series till the maximum arrives, and then the series begins again. No doubt when two waves coalesce they form a large one, but this does not occur at fixed intervals. The most common theory is that the tenth wave is the largest, but Tennyson says the ninth.
1
“And then the two
Dropt to the cove, and watch’d the great sea fall,
