Green’s Dictionary of Slang

(UK Und.) a thief, esp. an incompetent or novice one.

c.1698		B.E. Dict. Canting Crew n.p.: Dromedary, c. a Thief or Rogue [...] You are a purple Dromedary c. You are a Bungler or a dull Fellow at thieving.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Dromedary, a heavy bungling thief or rogue; a purple dromedary, a bungler in the art and mystery of thieving, (cant).
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1835		G. Kent Modern Flash Dict. n.p.: Dromedary, a clumsy thief, a young beginner.
1848		Flash Dict. in Sinks of London Laid Open.
1859		Matsell Vocabulum.
1881		Trumble Sl. Dict. (1890).