Green’s Dictionary of Slang

(orig. US drugs) marijuana.

1969		R.R. Lingeman Drugs from A to Z (1970).
1977		L. Heinemann Close Quarters (1987) 17: Smoke is M.J. Mike Juliet. Ya know — grass.
1989		P. Munro Sl. U.
2001		ONDCP Street Terms 14: M.J. — Marijuana.