The term "420" became associated with marijuana because of a group of five high school students in San Rafael, California, in 1971. These friends, called the Waldos, used to meet after school at 4:20 p.m. to search for an abandoned marijuana crop based on a treasure map. Although they never found the crop, they used "420" as a code to refer to smoking marijuana. The term spread through their acquaintances and was popularized by the followers of the Grateful Dead band, eventually becoming widely known in cannabis culture. The date April 20 (4/20 in U.S. date format) also became an annual celebration for cannabis enthusiasts to gather and honor the culture by smoking marijuana.

Several myths exist about the origin of 420, such as it being a police code or related to other cultural references, but the most credible and accepted story is the one about the Waldos and their 4:20 p.m. meetups.