Green’s Dictionary of Slang

applejack n.3

[rhy. sl. = crack n.7 (1)]

(drugs) crack cocaine.

[US]ONDCP Street Terms 2: Apple jacks — Crack Cocaine.
[Ire]Breen & Conlon Hitmen 221: ‘We’re gonna buy a nice bit of applejack [...] Applejack is a slang term for crack cocaine’.