Green’s Dictionary of Slang

1. a prison.

1974

(con. 1930s) A. Brink Looking on Darkness 54: The Juts sent me to the stokkies.

2. in sing. stokkie, a prisoner.

1987		T. Jenkin Escape from Pretoria 78: The political prisoners were allowed to enter the stokkies’ yard through a gate that was never closed, provided there were no stokkies in it. [Ibid.] 156: And if we were dressed in suitable clothing we would have looked like stokkies to the stokkies, not like escaping political prisoners.
2003		W. Ridgway Break Out 12: We lived on one side and the ‘stokkies’ lived on the other. Stokkies were not terrorists. They’d broken the law in other ways.