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To rump any one; to turn the back to him: an evolution sometimes used at court. Rump and a dozen; a rump of beef and a dozen of claret; an Irish wager, called also buttock and trimmings. Rump and kidney men; fiddlers that play at feasts, fairs, weddings, &c. and live chiefly on the remnants.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Rumford * RumpusNathan 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.