Green’s Dictionary of Slang

donkey dick n.¹

an ass.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Donkey, donkey dick. A he, or jack ass: called donkey, perhaps, from the Spanish or don-like gravity of that animal.
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum.