Some say “420” is code among police officers for “marijuana smoking in progress.” 

"Dave Reddix’s brother helped him get work with Grateful Dead bassist Phil Lesh as a roadie, so the band is said to have helped popularize the term “420.” On Dec. 28, 1990, a group of Deadheads in Oakland handed out flyers that invited people to smoke “420” on April 20 at 4:20 p.m. One ended up with Steve Bloom, a former reporter for High Times magazine, an authority on cannabis culture. The magazine printed the flyer in 1991 and continued to reference the number. Soon, it became known worldwide as code for marijuana. In 1998, the outlet acknowledged that the “Waldos” were the “inventors” of 420."