Green’s Dictionary of Slang

(UK Und.) sent to prison and thus forced to endure a diet of gruel; thus stun on skilly v., to endure a diet of gruel in prison.

1859, 1860		Hotten Dict. of Modern Sl. etc.
1861		Melbourne Punch ‘The Lay of the Lags’ 14 Mar. 1/1: Then when all the bobbies blewed are, / We can buz and rim a billy, / None to pall the trick and queer us, / Sending us to stun on skilly.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 236/2: He ‘made himself up,’ [...] as a bishop! and ‘did stunning, until he was afraid of being stunned on skilly’.
1864, 1867, 1870		Hotten Sl. Dict.