A ladder: perhaps from Jacob’s dream. CANT. Also the common name for a jay, jays being usually taught to say, Poor Jacob! a cup of sack for Jacob.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
jacob * jacobitesNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes