Green’s Dictionary of Slang

(US prison) a life sentence.

1950		Goldin et al. DAUL 18/1: All of it. [...] A life sentence to prison.
1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 197: all of it, n. – a life sentence.