Green’s Dictionary of Slang

(US drugs) marijuana, a marijuana cigarette.

1955		H. Braddy ‘Narcotic Argot Along the Mexican Border’ in AS XXX:2 87: LENO (Sp. for ‘piece of firewood’), n. A marijuana cigarette.
1971		E.E. Landy Underground Dict. (1972).
1992		Bentley & Corbett Prison Sl. 53: Leno (Pronounced lain-yo.) A marijuana cigarette or ‘joint.’.
2001		ONDCP Street Terms 14: Leno (Spanish) — Marijuana.