Definition of 420 in English:

420

Pronunciation /fôrˈtwen(t)ē/ /fɔrˈtwɛn(t)i/

noun

usually as modifier
  • 1US informal Used to refer to marijuana or to the act of smoking marijuana.

    • ‘a 420-friendly hotel’
    • ‘we already got the obligatory 420 joke out of the way’
    1. 1.1April 20th (4/20), considered as an occasion for smoking or celebrating the smoking of marijuana.
      • ‘thousands of marijuana smokers converged on Hippie Hill for the annual 420 celebration’
      • ‘Happy 420!’

Origin

1970s apparently coined as a code word by high school students in San Rafael, California in 1971 with reference to 4.20 p.m., the time at which they regularly met to try to locate an unattended plot of cannabis plants.