A polite term for the posteriors. Also, in the sporting sense, strength and spirits to support fatigue; as a bottomed horse. Among bruisers it is used to express a hardy fellow, who will bear a good beating.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Bottle-headed * Bottomless PitNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes