Green’s Dictionary of Slang

(US Und.) a life sentence; indefinite confinement on the punishment block.

1933		Ersine Und. and Prison Sl.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 74/2: From-now-on, n. 1. (P) An indefinite commitment to a segregation or punishment wing, often for the duration of a prison term. 2. (P) A life term or any equivalent prison sentence.