Back in the day in my hood we used to call it weed. We were all like, "Let's get some weed, man!" and "You wanna smoke some weed?" It was so hardcore.
Well obviously the nicknames are for situations where some stealth is required, like at work, bible study group, grandma's funeral, AA meeting, etc.
I thought I heard that 420 was California Penal Code for drugs?
I heard that too, but I read an article once where they couldn't find any record of it.
Some of my friends refer to it as "gardening".
i.e. "Man, later I'm gonna go gardening."
I used that one in a bookstore I worked at!