Green’s Dictionary of Slang

buck n.2

[cabman’s jargon buck, a fraudulent passenger used by an unlicensed London cabbie, who could get near a theatre or restaurant in the Strand only if he appeared to be bringing a fare; thus a reasonably respectable young man would be picked up, driven a few yards and then dropped]

an unlicensed cab-driver; orig. use seems to be as v.

[Ire]Freeman’s Jrnl 21 Sept. 1/5: I drives a cab, that is, I ‘bucks’ for the regular drivers ven they goes for to take a snooze for an hour or so.
[US]Matsell Vocabulum 15: buck A hack-driver.
[UK](con. 1840s–50s) H. Mayhew London Labour and London Poor III 352/2: The bucks are unlicensed cabdrivers.
[UK]Sl. Dict.
[UK]Farmer & Henley Sl. and Its Analogues.