Green’s Dictionary of Slang

1. (US short order) hot chocolate.

1966

in Newark (OH) Advocate 21 May 3/3–4: fifty-one —hot chocolate.

2. (drugs) crack cocaine.

2001

ONDCP Street Terms 9: Fifty-one — Crack.

3. (drugs) a cigarette made from a mix of marijuana and crack cocaine.

2001

ONDCP Street Terms 9: Fifty-one — [...] crack sprinkled on tobacco.