According to Steven Hager, editor of High Times, the term 420 originated at San Rafael High School, in 1971, among a group of about a dozen pot-smoking wiseacres who called themselves the Waldos, who are now pushing 50.
The term was shorthand for the time of day the group would meet, at the campus statue of Louis Pasteur, to smoke pot. Intent on developing their own discreet language, they made 420 code for a time to get high, and its use spread among members of an entire generation.
A quote from one of the Waldos in the High Times article states, "We did discover we could talk about getting high in front of our parents without them knowing by using the phrase 420."
One to grow on!
