Green’s Dictionary of Slang

dick n.2

[? link to a celebrated contemporary coachman Walter Dickson, nicknamed ‘Dickie the Driver’]

a riding whip.

’The Druid’ Post and Paddock 242: A legion of Hard-Riding Dicks are ever at work.
[UK]Hotten Sl. Dict.
[UK]Sl. Dict.
[UK]Farmer & Henley Sl. and Its Analogues.