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