Green’s Dictionary of Slang

1. a perjurer.

1725

J. Wild ‘Advice to his Successor’ in Fielding Hist. of Life of J. Wild (1840) lxvi: There are ways to bring honest men into scrapes, whereby they may, if the plumpers rap hard* come in for a scragging bout (*Fellows hired to swear; Keep close what they swear).

2. a single vote at an election.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1790		J. Freeth ‘The Female Canvasser’ Political Songster 117: He [...] boldly sung ‘I give Charles Fox a plumper’.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1838		D. Jerrold Men of Character III 280: I’ve no manner of doubt that every one of the ‘brethren’ got Sir Jeremy ten plumpers.
1843		Comic Almanack Aug. 374: Both must have received a few plumpers, and the state of their respective polls must be rather unsatisfactory.
1859, 1860		Hotten Dict. of Modern Sl. etc.
1860		E. Eden Semi-Attached Couple (1979) 227: The black lips opened in answer to the interrogation of the polling-clerk, and announced a plumper for Colonel Beaufort.
1872		‘The Right Tap’ Fun July n.p.: If the lever, meaning a plumper, were labelled ‘stout,’ and those recording a split vote half and half, the illusion would be complete [F&H].
1885		Bulletin (Sydney) 31 Oct. 12/4: ‘Did you vote yesterday, Pat?’ ‘I did, sorr, and guv you a plumper. I put a big shtroke acrass yer name, and left the other dirhy blaggards out in the crowd!’.
1894		J.K. Fowler Recollections Old Country Life 8: An old printed document [...] giving [...] the number of plumpers, or single votes, polled for each candidate .
1895		C. Crowe Aus. Sl. Dict. : Plumper, a single vote and no‘splitting’.
1914		C. Holme Lonely Plough (1931) 180: He says those ten female plumpers could have put you at the head of the poll.

3. a heavy bet.

1984

Partridge DSUE (8th edn) 901/1: from ca. 1881.