Green’s Dictionary of Slang

(UK prison) a bath.

1892

Sporting Times 9 Apr. 1/3: Voice of gentleman describing himself as a Toke and Streamy Merchant [...] ‘Bill, d’you know the [House of] Detention skilly? Aw’fly rich in flour [...] you can smell it as soon as you come out o’ the plaster-an’-lath’.