Green’s Dictionary of Slang

1. (N.Z. prison) a heavily decorated and equipped cell, usu. that of a long sentence inmate.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 138/2: penthouse n. 1 a cell, esp. belonging to a lifer, as it may contain more possessions than other inmates’ cells (e.g. an aquarium, a bird in a cage, general furnishings) given that its occupant is a long-term resident of the prison.

2. (N.Z. prison) to top landing of a wing or a cell on that landing.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 138/2: penthouse n. 2 the top landing of a multistoreyed wing; a cell on this landing.