Rumored to have been started from either a secret police code, the specific time of day for tea in Amsterdam, or even Hitler's birthday, the real reason behind 420's special meaning for cannabis has always remained a mystery. Even Warren Haynes, a touring member of the remaining Grateful Dead group, had no idea when asked.
As it turns out, the origin of 420 actually begins with the Grateful Dead. First sparked by a flyer for a concert in 1990, the meeting time (which read 4:20 on 4/20 for 420-ing) was one of the first times anyone had ever used the number as a synonym for marijuana. However, the flyer provided its own background information on how 420 had come to be for them as well. "420 started somewhere in San Rafael, California in the late '70s. It started as the police code for Marijuana Smoking in Progress. After local heads heard of the police call, they started using the expression 420 when referring to herb -- Let's Go 420, dude!"
The Grateful Dead comes into play as the ‘70s hippie era faded and counterculture began to rise. The band toured through the end of the 80s and spread the term they came across when playing parking lot concerts in NorCal. As cannabis publications like High Times began gaining traction, the term began to take off as well. Editors used it frequently when discussing cannabis events like Cannabis Cup and the World Hemp Expo, allowing it to quickly become an international phrase.
However charming the flyer's story reads, further research unveiled that only part of the story was true. 420 did originate in San Rafael, but it was actually through a group of five friends who called themselves The Waldos that named named 420 in 1971.
While High Times took the term and made it a phenomenon, The Waldos remain the group who coined the term “420”. Unlike High Times, which bought the domain 420.com in the '90s, The Waldos never made any money from the term.