Green’s Dictionary of Slang

contraband, smuggled; made in an illicit still.

1866		Sportsman 22 Dec. 2/1: Notes on News [...] We have heard of ‘jiggered gin’ poisonously strong enough make a Whitechapel rough ‘run a muck’.
1886		Judy 4 Aug. 58: He imbibed stupendous quantities of jiggered gin, dog’s nose, and Paddy’s eye-water [F&H].