Hanover jack, n. — Green’s Dictionary of Slang

Hanover jack n.

[? counterfeit sovereigns produced in Germany and bearing the head of James II or Jac(obus); they were infiltrated into England and circulated during the reign of William III (r.1689–1702)]

an imitation sovereign.

1884		Sporting Times 1 Nov. 1/4: Q. What is the national currency? A. [...] Hanover Jacks, snide white ’uns, duffing browns, flash flimsies, stumers, bits of stiff, kites, tombstones.
[1888		police report in Ware (1909) 150/1: On searching the prisoner I found twenty-five ‘To Hanover’ sovereigns usually carried by magsmen, several ‘Bank of Engraving’ notes, and two duplicates relating to coats].
1890		‘Morris the Mohel’ ‘Houndsditch Day By Day’ in Sporting Times 11 Jan. 3: Here, servelp me, vhat’s this?—somepody’s run in two ’Anover Jacks on me!
1900		Sporting Times 10 Feb. 2/2: No stumers, Hanover Jacks, clogs, stools, umbrellas, pinched prices [etc.].