[Called in some counties a leaping stock, in others a jossing block.] Steps for mounting a horse. He sits like a toad on a jossing block; said of one who sits ungracefully on horseback.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Upper Story * UppishNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes