Green’s Dictionary of Slang

1. (US black/drugs) heroin.

1996

(con. 1980s) L. Pettiway Honey, Honey, Miss Thang 211: When I met Kane I started doing heroin. I snuck behind his back and tried it ’cause they used to call it apple back then [...] and I wanted to see what apple was about.

2. (UK prison) the situation, the ‘score’.

2004

(con. 1990s) N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 413: I [...] spoke to him through the gap between the pipe and the wall. ‘Danny, what’s the apple?’.