Green’s Dictionary of Slang

a shilling (12 old pence/5p).

1850		H. Mayhew in Advocate 18 Dec. 10/1: ‘I’ll back Jem for a yanepatine,’ says one. ‘Jack for a gen,’ says another.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 17/2: ‘I’ll try you a “gen” (shilling)’ said a coster.
1873		N.S. Dodge ‘Vagrants and Vagrancy’ in Appleton’s Journal (N.Y.) 6 Sept. 308: Vagrant vernacular [...] is not remarkable for originality, and, unlike most low languages, possesses no spice of humor. The main principle upon which this peculiarity revolves, is to spell and pronounce the substantives in any sentence backward. [...] Gin becomes shilling, and owt gins two shillings. Yenork is a crown, and deenop a pound.
1881		Morn. Bulletin (Rockhampton, Qld) 18 July 2/6: For a shilling there are many names but nearly all slang. [...] ‘Breaky-leg,’ ‘brongs,’ ‘bobs,’ ‘bordes,’ ‘drawers,’ ‘gens’, ‘hogs,’ levys,’ ‘pegs,’ ‘stags,’ ‘Shigs,’ ‘twelvers’ and ‘teviss’s’ .
1890		Northampton Mercury 12 Apr. 12/4: The word ‘generalise’ is supposed to be ‘shilling’ spelt backwards, and this is abbreviated into ‘gen,’ the coster’s equivalwent of the Cockney’s ‘bob’.
1896		‘Thieves’ Sl.’ Gent.’s Mag. CCLXXXI Oct. 348: A ‘gen’ [...] would stand for a shilling.
1925		(ref. to 1850s) Western Mail (Perth) 28 May 21/1: [from Daily Mail, London]At the time of the Crimean War bob was only one of a number of terms [for a shilling] such as twelver and breaky-leg, gen and teviss, stag, deaner, hog and levy.
1935		R.T. Hopkins Life and Death at the Old Bailey 656: Gen is a shilling.
1953		J. Franklyn Cockney 297: The back-spelling of ‘shilling’, being unpronounceable, is rendered generalise and abbreviated to gen.