Green’s Dictionary of Slang

rock adj.

1. excellent, outstanding. [i.e. it knocks one off-balance].

[US]D. Burley Orig. Hbk of Harlem Jive 146: Rock — Supreme pleasure, tops in dancing and music, finest esthetic delight.
[US]Eble Campus Sl. Mar. 7: rock – cool, awesome, or totally superior.
[US]Da Bomb [Internet] 24: Rock: 1. Elite; cool; fun; exciting; loud.
[UK]T. Black Gutted 84: I’m rock. After the people-smuggling bust I’m...well, I’m solid.

2. very firm, adamant, uncompromising [SE rock-hard].

[UK]K. Sampson Outlaws (ms.) 40: Membership’s rock though, Ray says. They’re strict as fuck who they’ll let in.