420

In 1971, five high school students – Steve Capper, Dave Reddix, Jeffrey Noel, Larry Schwartz, and Mark Gravich – in San Rafael, California, calling themselves the Waldos because "their chosen hang-out spot was a wall outside the school", used the term in connection with a 1971 plan to search for an abandoned cannabis crop that they had learned about, based on a treasure map made by the grower. The Waldos designated the Louis Pasteur statue on the grounds of San Rafael High School as their meeting place, and 4:20 p.m. as their meeting time. The Waldos referred to this plan with the phrase "4:20 Louis". After several failed attempts to find the crop, the group eventually shortened their phrase to simply "4:20", which ultimately evolved into a code-word that the teens used to mean consuming cannabis.

Mike Edison says that Steven Hager of High Times was responsible for taking the story about the Waldos to "mind-boggling, cult-like extremes" and "suppressing" all other stories about the origin of the term. Hager wrote "Stoner Smart or Stoner Stupid?", in which he attributed the early spread of the phrase to Grateful Dead followers – after Reddix became a roadie for the Dead's bassist, Phil Lesh – and called for 4:20 p.m. to be the socially accepted hour of the day to consume cannabis.

So far the majority of you are incorrect.
420's origin IS known.
the term 420 originated at San Rafael High School, in 1971, among a group of about a dozen pot-smoking wiseacres who called themselves the Waldos, who are now pushing 50. The term was shorthand for the time of day the group would meet, at the campus statue of Louis Pasteur, to smoke pot. Intent on developing their own discreet language, they made 420 code for a time to get high, and its use spread among members of an entire generation.
So there ya go, someone's parents out there invented the term 4/20.
And remember this:
There are NOT 420 chemicals in Weed. It's about 315, the num. goes up or down depending on what you're smoking.
4/20 is NOT police code or Maryjane.
And that whole shit about Holland and 4/20 over there being "tea time" for smokers isn't true either.

I know my shit.

In 1971, a group of stoners at San Rafael High School, California, coined this word. It was originally 420 Louis, meaning they planned to smoke weed by the Louis Pasteur statue by their school (btw Louis Pasteur was a hygene expert just fyi) and they said it as like a code word so that they could talk about weed around their parents / teachers and they would not know what the hell they were going on a bout!!! They may have been trying to track down a hidden crop aswell, although that is a bit debated and may be in fact an urban myth. But hey, this is Urban Dictionary, so yea! And then the phrase gained use thanks to the Greatful Dead who publicised it in the 70s and 80s. Today, 44 years down the track, the Waldos are given full credit for coining the term and it still means weed. It is mainly used for fun, secrecy, ritual and protest. A lot of people who smoke weed have a lot of fun with this word, and nobody else really knows wtf they are saying! ... (except their hippy parents!!!!!!! lol oops!)

Common misconceptions point to crazy theories about weed but the truth is it is actuly a number between 419 and 421.

420 is 42 the meaning of life some say but a 0 added to the end the 0 is post to show weed. 420 can be used to like smoke weed or something

420, indeed, was thought of years ago as the time of day some people got together to smoke pot. Now it is used as code for "Let's go get high." Also, some people consider it to be April 20th that is "National Pot Smoking Day."
I prefer all of the above.

To add to the definition regarding "The Waldos," I'd like to note that in an article of High Times containing an interview with the waldos, one of them told this story:
We had heard rumors that one of the members brothers had planted a pot patch in the wooded area around town. We all wanted to go look for this patch of weed, so we devised to meet eachother by the statue of Louis Pasteur after school. Since we all had after school activities or electives that lasted about fifty minutes, and since it took about ten minutes to get from school to our after school activities, and another ten minutes to walk from the activites to the statue. Since there were many of us we decided that 420 would be a good time to meet so we could go search for the patch of pot. We never did find it, but we had lots of fun getting high while trying to look for it.