killer weed, n. — Green’s Dictionary of Slang

killer weed n.

[SE killer + weed n.¹ (1d); orig. a non-sl. epithet applied to discourage use, now used ironically]

(drugs)

1. marijuana.

1971		B. Moyers Listening to America 370: You’re not giving me the line that marijuana is indeed the killer weed?
1999		‘Master Pimp’ Pimp’s Rap 1: I had just got through smoking five joints of that killer weed.

2. phencyclidine.

1978		Daily News 23 Feb. 5: Angel dust [...] also is called killer weed because it causes numbness in the arms and legs and because it makes some users feel ‘dead’.
1981		D.E. Miller Bk of Jargon 338: killer weed: PCP.
2001		ONDCP Street Terms 13: Killer weed (1980’s) — Marijuana and PCP.