Green’s Dictionary of Slang

1. a military guardroom, a county or city prison.

1877		H. Hartigan Stray Leaves (1st ser.) 128: [T]he ‘clink,’ ‘the digger,’ ‘the corner shop’ or any of the thousand and-one names whereby the men designate the guard-room.
1896		‘Army Slang’ in Regiment 11 Apr. 31/1: The guard-room (prisoners’ room) is [...] the ‘net,’ ‘trap,’ ‘clink,’ ‘dust-hole,’ ‘cage,’ ‘digger,’ ‘dog’s-home, ’ ‘marble-arch’.
1900		Regiment 27 Jan. 288/1: [W]hen a soldier [...] is placed in the guard-room for being drunk or any other offence, he is said to have been placed in the ‘digger’ or ‘corner shop,’ or the ‘mush’ .
1920		P. MacGill Fear in DSUE (1984).

2. a solitary confinement cell.

1953		I. Hamilton Till Human Voices Wake Us 64: ‘You’re in the dummy, down in the pound, the digger, whatever you like to call it. You've started doing three days solitary confinement on bread and water’.
1971		T. Shadbolt Bullshit and Jelly Beans 10: Another feature of prison life of which all men are aware is ‘the digger’. Solitary confinement. One visit a fortnight, one letter a week, very few blankets.
1982		G. Newbold Big Huey 89: The screws held back, and hustled Jake away to the digger.
1992		Bentley & Corbett Prison Sl. 10: Damper also Digger The segregation unit or hole.
1998		R. Bungay Scarecrows: Why Women Kill 126: I was in hell when I first went into prison [...] I spent day after day in the digger because of my own behaviour.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 55/2: digger, the n. the solitary confinement punishment cell [...] 192/1: top digger, the n. see digger, the.