Green’s Dictionary of Slang

1. a prisoner.

1980

E. Leonard Gold Coast (1983) 16: Little guinea trying to sound like a hard-timer, bit off words barely moving his mouth.

2. a prisoner who suffers in prison (whether from the regime or from self-inflicted problems).

1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 206: hard timer, n. – one who breaks many rules and takes much punishment; also someone who is homesick.
1998		L. Stringer Grand Central Winter 83: [A] ragtag collection of druggies, winos, petty hustlers, panhandlers, and low-level dealers overseen by a hard-timer working off the terms of his parole.