Green’s Dictionary of Slang

(US black) a tough person, both emotionally and physically.

1927		F.M. Thrasher Gang 222: The leader of our gang was what is usually termed a ‘hard rock.’ He was the leader because he was the ‘hardest’ .
1956		‘Ed Lacy’ Men from the Boys (1967) 28: What’s all this about [...] upset stomach? According to my records you’re a typical hard rock.
1961		J. Scarne Complete Guide to Gambling 681: Hard rock – 2. A gambler who refuses to lend money. 3. A player who is hard to beat.
1968		T. Wolfe Electric Kool-Aid Acid Test (1969) 64: The hard rocks are hunkered down telling jokes about pussy.
1988		Ice-T ‘Heartbeat’ 🎵 That’s who I made it for, the hardrocks.
1992		S.L. Hills Tragic Magic 145: I’ve seen a change from the James Cagney days when you had to be hard rocks.
1995		L. Stavsky et al. A2Z.
2008		‘Grandmaster Flash’ Adventures 24: ‘Nearly every kid my age was running with a gang [...] They were wall-to-wall hardrocks, lookin’like black and Puerto Rican Hell’s Angels’.