Green’s Dictionary of Slang

1. a guinea.

1899		C. Rook Hooligan Nights 63: The toff [...] give me a jimmy.
1909		J. Ware Passing Eng. of the Victorian Era 160/1: Jimmies (Hist., 17 cent.). Guineas – in the reign of James II. Remains to this day.
1928		M.C. Sharpe Chicago May (1929) 42: The gentleman slipped the flunky a half-Jimmie (half-sovereign), and he never batted an eye, though I could see he knew me.

2. a sovereign (£1.00); thus half-(a)-jimmy, 10/-.

1887		Bulletin (Sydney) 29 Oct. 7/4: Three sweet burglars in Melbourne are in a tight place. The old story. Too fond of the Jimmies.
1898		Hartlepool Mail 26 Feb. 6/4: Jemmy [sic], a sovereign.
1899		A. Binstead Gal’s Gossip 17: He dragged the half-jimmy — his little ewe-lamb! — out of his jeans.
1901		Truth (Sydney) 10 Nov. 7/2: He is fined a half a jimmy.
1901		Sporting Times 20 Apr. 1/4: He’d withdrawn [...] from the trusting maiden’s purse two half-Jimmies and a ‘sprarzer’.
1913		Truth (Sydney) 5 Jan. 9/5: ‘I did part ten golden jimmeys / For them chips as I did win’.
1913		Sport (Adelaide) 22 Mar. 12/4: They Say [...] That Wicker H found half a jimmy [...] and bought a collar on the strength of it.
1919		‘Sax Rohmer’ Dope 139: ‘Ten-six Smyrna; one ’leben Patna,’ muttered Sin Sin Wa. ‘You catchee eighty jimmies.’.