Green’s Dictionary of Slang

generalize n.

a shilling.

1859		Hotten Dict. of Modern Sl. etc. 127: Generalize a shilling, generally shortened to gen.
1860		Hotten Dict. of Modern Sl. etc. [as cit. 1859].
1890–1904		Farmer & Henley Sl. and Its Analogues.
1953		J. Franklyn Cockney 297: The back-spelling of ‘shilling’, being unpronounceable, is rendered generalise and abbreviated to gen.