Green’s Dictionary of Slang

(UK Und.) aqua fortis, nitric acid when used to test for gold.

1935		G. Ingram Cockney Cavalcade 120: ‘Try the acky on it if you like. There y’are,’ he went on, putting the acid on the table, together with a file.
1953		J. Phelan Underworld 20: The acky is acquafortis wth which a fence wll test every mllmetre of every article alleged to be gold.
1972		P. Fordham Inside the Und. 45: The ‘acky’ trick – you know, switching the jar of nitric acid ‘aqua fortis’ [...] for another.