Green’s Dictionary of Slang

plumper n.2

[SE plump for, to commit wholeheartedly]

1. a perjurer.

[UK]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.

[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]J. Freeth ‘The Female Canvasser’ Political Songster 117: He [...] boldly sung ‘I give Charles Fox a plumper’.
[UK]Lex. Balatronicum.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
[UK]D. Jerrold Men of Character III 280: I’ve no manner of doubt that every one of the ‘brethren’ got Sir Jeremy ten plumpers.
[UK]Comic Almanack Aug. 374: Both must have received a few plumpers, and the state of their respective polls must be rather unsatisfactory.
[UK]Hotten Dict. of Modern Sl. etc.
[UK]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.
[UK] ‘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].
[Aus]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!’.
J.K. Fowler Recollections Old Country Life 8: An old printed document [...] giving [...] the number of plumpers, or single votes, polled for each candidate .
[Aus]C. Crowe Aus. Sl. Dict. : Plumper, a single vote and no‘splitting’.
[UK]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.

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