Green’s Dictionary of Slang

(UK prison) a prisoner who is generous in bribing warders; one who is easily tricked out of money or property.

c.1698		B.E. Dict. Canting Crew n.p.: Milch-kine, a Term us’d by Goalers, when their Prisoners will bleed freely to have some Favor, or be at large.
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.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859		Matsell Vocabulum 54: milch cow A man that is easily cheated out of his money.