Green’s Dictionary of Slang

of a villain or thief, professional, i.e. so hardened as to be beyond shame.

c.1698		B.E. Dict. Canting Crew n.p.: Scandal-proof, a thorough pac’d Alsatian or Minter, one harden’d or past shame.
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.: Scandalproof, one who has eaten shame and drank after it, or would blush at being ashamed.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859		Matsell Vocabulum.
1895		C. Crowe Aus. Sl. Dict. 70: Scandal Proof, shameless.