Green’s Dictionary of Slang

1. (UK Und.) a prison or police station.

1902

A.N. Lyons Hookey 51: The jug – the clink – the lock-up – the rest-house – the King’s waiting room – the – in short, the police-station.

2. (US Und.) any prison where the discipline is lax and the work undemanding.

1927		‘Jargon of the Und.’ in DN V 461: Rest house, An easy penitentiary.
1931		Irwin Amer. Tramp and Und. Sl. 158: Rest House.–A prison or other penal institution where the work is easy and the discipline not severe.