Green’s Dictionary of Slang

1. a coach; cit. c.1800 may refer to the hangman’s tumbril.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Dilly, 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.
1793		Sporting Mag. Dec. III 163/1: Would you ride in the caravan, the expedition, the land frigate, or the dilly.
c.1800		‘The Rolling Kiddy’ in Holloway & Black I (1975) 234: The morning dilly puts a stop to such a rolling tippy O.
1818		‘Thomas Brown’ Fudge Family in Paris Letter X 116: Beginning gay, desperate, dashing, down-hilly, / And ending as dull as a six-inside Dilly.
1834		Marryat Peter Simple (1911) 50: We found all sorts of vehicles ready to take us to the fair. We got into one which they called a dilly. I asked the man who drove it why it was so called, and he replied, because he only charged a shilling.
1838		Crim.-Con. Gaz. 29 Sept. 47/1: [of a French diligence] [I] determined to forswear the Mall Post, and go to Paris in the Dilly.
1861		R.F. Burton City of the Saints 247: The people came [...] to see the mail-coach, as if it were the ‘Derby dilly’.

2. a night-soil cart.

1873

Sl. Dict.