Green’s Dictionary of Slang

(UK Und.) a cloak.

c.1566		Harman Caveat for Common Cursetours in Viles & Furnivall (1907) 82: a caster, a cloke.
1592		Groundworke of Conny-catching n.p.: [as cit. c.1566].
1610		Rowlands Martin Mark-all 42: My Caster and Commission shall serue us both to maund.
1637		Dekker Canting Song in Eng. Villainies (8th edn) O2: Though I no Caster wore, nor Commission, no nor slate.
1648		Dekker Canters Dict. Eng. Villainies (9th edn).
1688		R. Holme Academy of Armory Ch. iii item 68c: Canting Terms used by Beggars, Vagabonds, Cheaters, Cripples and Bedlams. [...] Caster, a Cloak.
c.1698		B.E. Dict. Canting Crew.
1725		New Canting Dict.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict.
c.1750–1882		B.M. Carew Life and Adventures.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1832		Egan Bk of Sports 199: Neal [...] gently dropped his caster within the ropes.
1845		Bell’s Life in Sydney 6 Sept. 4/3: Bungaree walked quietly to the edge of the ring, and threw in his caster, immediately after introducing his own person between the ropes.
c.1850		Duncombe New and Improved Flash Dict.
1859		Matsell Vocabulum.