Sham or collar shirts. Also partizans for the Stuart family: from the name of the abdicated king, i.e. James or Jacobus. It is said by the whigs, that God changed Jacob’s name to Israel, lest the descendants of that patriarch should be called Jacobites.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Jacob-1 * JadeNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes