Wiki and several other places say this:
The term was allegedly coined by a group of teenagers in San Rafael, California in 1971.[2][3] Calling themselves the Waldos, because "their chosen hang-out spot was a wall outside the school,"[4] the group first used the term in connection to a fall 1971 plan to search for an abandoned cannabis crop that they had learned about.[5] The Waldos designated the Louis Pasteur statue on the grounds of San Rafael High School as their meeting place, and 4:20 p.m. as their meeting time.[4] The Waldos referred to this plan with the phrase "4:20 Louis". Multiple failed attempts to find the crop eventually shortened their phrase to simply "4:20", which ultimately evolved into a codeword the teens used to mean pot-smoking in general.[5]
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