Green’s Dictionary of Slang

plump in the pocket adv.

satisfactorily well-off.

c.1698		B.E. Dict. Canting Crew n.p.: Plump-in-the-Pocket flush of Money.
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 68: plump Rich; plenty of money. ‘A plump skin,’ a full purse.
1895		C. Crowe Aus. Sl. Dict. 59: Plump, plenty of money.