Green’s Dictionary of Slang

plump v.

to hit, to shoot; also fig. use.

1754		G. Stevens ‘A Cant Song’ Muses Delight 177: We fil’d the rum codger and plumpt the queer cull, / And away we went to the ken boozie.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Plump [...] To plump; to strike, or shoot. [...] He pulled out his pops and plumped him; he drew out his pistols and shot him.
1794		Sporting Mag. Oct. V 6/1: My Kitty, who, if any man does but squint at, I’ll plump and rib him.
1802		G. Colman Yngr Poor Gentleman III i: Give me a man who is always plumping dissent to my doctrines smack in my teeth.
1811		Lex. Balatronicum [as cit. 1785].
1821		R. Waln Hermit in America on Visit to Phila. 2nd series 107: He must be a ‘bruiser’withal, and [...] mill his man, fib his nob, spill claret, darken day-lights and plump peepers!
1837		A. Greene Glance at N.Y. II ii: I was a-tryin’ to plump one of dem saucy newsboys.
1851		‘Bainbridge’s Tid-Re I’ in Jack Tar’s Songster 16: O, swamp it, if you had only seen how we plumped her [...] and how our grape-shot rattled in at her port-holes.
1909		L.W. Payne Jr ‘Word-List From East Alabama’ in DN III:v 358: plump, v. To hit squarely.

In phrases

plump someone up to (v.) [they are ‘plumped up’ with the information]

to tell someone something secretly.

1923

J. Manchon Le Slang.