nounusually as modifier
1US informal Used to refer to cannabis or to the act of smoking cannabis.
- ‘a 420-friendly hotel’
- ‘we already got the obligatory 420 joke out of the way’
- 1.120th April (abbreviated 4/20 in the US), considered as an occasion for smoking or celebrating the smoking of cannabis.
- ‘thousands of cannabis smokers converged on Hippie Hill for the annual 420 celebration’
- ‘Happy 420!’
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.