[An abbreviation of the word diligence.] A public voiture or stage, commonly a post chaise, carrying three persons; the name is taken from the public stage vehicles in France and Flanders. The dillies first began to run in England about the year 1779.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Diligent * DimberNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes