E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A dummy fare who enables a cabman to pass police-constables who prevent empty cabs loitering at places where cabs will be likely to be required, as at theatres, music-halls, and large hotels. A cabman who wants to get at such a place under hope of picking up a fare gives a buck a shilling to get into his cab that he may seem to have a fare, and so pass the police.
Constables are stationed at certain points to spot the professional buck-riders.Nineteenth Century (March, 1893, p. 576).