Green’s Dictionary of Slang

mourner’s bench n.

[SAmE mourner’s bench, a bench set up at revival meetings for those ‘in mourning for their sins’]

1. (US) the court bench on which prisoners sit awaiting trial.

[US]CharlotteObserver (NC) 27 July 4: In the city police court a motley crowd of prisoners filled the mourners’ benches .
[UK]D. Lowrie My Life in Prison 358: The boy seated himself on the ‘mourner’s bench’, and the Captain went into his office.
[US]A.J. Pollock Und. Speaks.

2. (US prison) a bench on which new inmates sit, e.g. for indoctrination talks.

[UK]D. Lowrie My Life in Prison in Hamilton Men of the Und. 246: I sat on the ‘mourner’s bench’ and he stood before me, instructing me as to the rules.
[US](con. 1900s) S. Lewis Elmer Gantry 29: Nerve of him, trying to lead me up to any mourners’ bench!