half-dollar, n.² — Green’s Dictionary of Slang

half-dollar n.²

[SE half + fig. use of the whole SE dollar to mean 100 (cents/years) or 12 months]

1. (US prison) one who is serving 50 years in jail.

1971–2

C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 205: half dollar, n. – a man who is doing a fifty-year sentence.

2. (N.Z. prison) a six months sentence.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 83/1: half a dollar n. 1six months’ imprisonment.