E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The common legend is, that when Tromp, the Dutch admiral, appeared on our coast, he hoisted a broom on his ship, to signify his intention of sweeping the ships of England from the sea; and that the English admiral hoisted a horsewhip to indicate his intention of drubbing the Dutch. According to this legend, the pennant symbolises a horsewhip, and it is not unfrequently called the whip.