Having large projecting buttocks: from their resemblance to a small basket, called a hopper or hoppet, worn by husbandmen for containing seed corn, when they sow the land.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Hopping Giles * HornsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes