Dictionary.com
definitions

420

or 4/20,or 4:20

[fawr-twen-tee, fohr‐]
noun Slang
  1. marijuana: Are you carrying any 420 on you?
  2. marijuana drug use: Police presence at the concert discouraged 420.The guys at the party were all 420-friendly.
  3. the twentieth day of the fourth month, or the time 4:20, when referenced as a day or time for cannabis consumption or the celebration of marijuana culture: The head shop has a big pipe sale every year on 420.

Origin of 420

First recorded in 1970–75; a code word derived from the meeting time of a particular group of marijuana smokers in 1971, later popularized by fans of the band the Grateful Dead
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018