Green’s Dictionary of Slang

jacobus n.

a guinea.

1614		Jonson Bartholomew Fair IV iv: Can you lend me a piece, a Jacobus.
1663		Wandring whores complaint 4: I met with a Country Hick, and steping before him, I dropt a Jacobus, and taking it up, he cry’d half.
1728		J. Dalton Narrative of Street-Robberies 12: They took from this Woman, a Jacobus, a Gold Ring, and some Silver.
1734		C. Johnson Hist. of Highwaymen &c. 99: I got one Jacobus back again.
1753		J. Poulter Discoveries (1774) 10: We found in it the following [...] Eighteen Guineas, a Jacobus, and several other Pieces of Gold and Silver Coin.
1764		K. O’Hara Midas III ii: Count ’em – six guineas and an old jacobus.
1822		(con. early 17C) W. Scott Fortunes of Nigel I 309: He makes them a free present of the dinner and wine [...] and they, in gratitude, make the chevalier a present of a Jacobus.
1863		(con. mid-18C) G.A. Sala Strange Adventures of Captain Dangerous 236: To each of the dozen Warders there present he gave a Jacobus.