Green’s Dictionary of Slang

1. a cigarette.

1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 55: root, n. A cigarette.
1942		Berrey & Van den Bark Amer. Thes. Sl.

2. (drugs) marijuana.

1959		J.E. Schmidt Narcotics Lingo and Lore.
1969		R.R. Lingeman Drugs from A to Z (1970) 221: root [...] A marijuana cigarette.
1975		Wentworth & Flexner DAS.
1982		Abel Marihuana Dict.
2001		ONDCP Street Terms 18: Root — Marijuana.