four-twenty, n. — Green’s Dictionary of Slang

four-twenty n.

also 4:20

[email on American Dialect Society List 10/5/2001: ‘[The term] originated at San Rafael High School in San Rafael, CA, in the early 1970s as a code for smoking marijuana at 4:20 pm (70 minutes after school dismissal). The Grateful Dead were long based out of San Rafael, and the phrase was used on a flyer at Grateful Dead shows in 1990, leading to its wider use.’]

(US black/drugs) marijuana.

1974		Red & White (San Rafael High School, Marin Co, CA) 7 June 3/2: If you had the opportunity to say anything in front of the graduating class, what would you say [...] 4-20 [OED].
1997–2000		College Sl. Research Project (Cal. State Poly. Uni., Pomona) 🌐 420 (noun) Smoking weed. [Oxnard High School, Oxnard, CA; and Da Bomb].
1998		Eble Campus Sl. Apr. 11: 4.20 – pertaining to marijuana.
2002		L.A. Times 20 Apr. n.p.: ‘Four-twenty’ — once an obscure Bay Area term for pot — is showing up nationally in the advertisements and business names of concert promoters, travel agencies, even high-tech companies.
2002		Teen Lingo: The Source for Youth Ministry 🌐 4:20 (420) (4-20) n. Commonly known as the time to smoke pot. It has come to mean everything from the act of smoking, the stuff that’s smoked, and the optimum smoking time. Because 99% of today’s culture DOESN’T know what 420 means, it has become a code people use to identify and talk with each other without outsiders knowing. Also known for the date, April 20th, which is the day to smoke pot all day, ‘the hippie holiday.’.