A piece of whalebone or ivory, formerly worn by women, to stiffen the forepart of their stays: hence the toast—Both ends of the busk.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Bushel Bubby * Buss BeggarNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes