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Fat, full, fleshy. Plump in the pocket; full in the pocket. To plump; to strike, or shoot. I’ll give you a plump in the bread basket, or the victualling office: I’ll give you a blow in the stomach. Plump his peepers, or day-lights; give him a blow in the eyes. He pulled out his pops and plumped him; he drew out his pistols and shot him. A plumper; a single vote at an election. Plump also means directly, or exactly; as, it fell plump upon him: it fell directly upon him.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Plummy * Plump CurrantNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes
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Francis Grose was independently wealthy, having inherited money from his father, a jeweller. Finding himself overspending, he published a number of books; his Provincial Glossary seems to have been the starting-point for the Vulgar Tongue reproduced here.