Green’s Dictionary of Slang

(Aus./N.Z./UK prison) solitary confinement in prison.

1862		Cornhill Mag. vi 640: Criminals [...] in prison and out of it... doing their separates at Pentonville and among the rocks of Gibraltar [...] [F&H].
1877		Five Years’ Penal Servitude 333: A new large prison at Dartmoor, in which convicts could be confined in cells to do their separates, as the first eleven or twelve month’s probationary imprisonment is termed.
1885		M. Davitt Leaves from a Prison Diary I 162: Doing his ‘separates’ in Millbank for the third time.
1918		V. Marshall Jail From Within (1969) 33: With fourteen or fifteen other prisoners also doing ‘separate’, I [...] was led out.
1924		S. Scott Human Side of Crook and Convict Life 45: Taken [...] to do what is called ‘Separates’ — that is, solitary confinement.
1926		N. Lucas London and its Criminals 149: After doing his ‘separates’ (solitary confinement) at a local prison.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 162/1: separates (also seps) n. punishment segregation.