(A). 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).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Buck-rider from Infoplease: