Green’s Dictionary of Slang

(Aus./N.Z. prison) that area of the prison reserved for intractable prisoners, the solitary confinement cells.

1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Bronx. Area of a cell block where the intractable or ‘heavy’ prisoners are located.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 31/1: Bronx, the n. 1 the solitary confinement punishment cell, the pound. 2 the wing of a prison heavily populated by gangmembers.