Are you wondering what is 420 slang for? Well, the answer is quite simple, but the implications may be greater than you would imagine.

420 is slang for marijuana usage. People who smoke pot often make jokes when the clock reaches 4:20, or they slyly say that they are going to participate in 420 today. If you hear anyone talking about 420 in an at least somewhat secretive way, he or she is probably talking about going to smoke pot.
However, 420 is not as underground as some other terms for pot or more so, other terms for different drugs. Not everyone takes marijuana usage seriously as the usage of other drugs, such as ecstasy. Therefore, people do not necessarily try as hard to hide their marijuana usage.
Lots of people have their own theories about where 420 came from. However, most of them are incorrect. The term 420 orginated at San Rafael High School in 1971. There was a group of pot smokers who called themselves the Waldos. They were not a large group; there were only about twelve or so of them. However, they would meet at 4:20 to smoke pot, by the statue of Louis Pasteur, located on campus.
After that, they started using the code "420" for any time that they would want to go and smoke pot. However, its origins were a very specific time at a specific location.
A number of misconceptions exist revolving around the question of what is 420 slang for.
420 is not the only slang term for marijuana. In fact, when people are talking about marijuana, they hardly ever use the word "marijuana." The most frequently used words are both weed and pot. Most of the time, 420 is used if someone is talking about an event, or talking about the actual date. As mentioned earlier, it's often used in a more joking sense too. People usually do not say "Let's go do some 420." They will say "Who has the pot?" or "Where's the weed?"