Is cannabis/marijuana really a weed?

Weed is slang. I don't mean that in the way we all know, like slang for marijauna, i.e. pot, whatever. But weed is a slang. It is a meaningless botany term. It just means an undesirable plant. So any plant can be a weed. Marijuana isn't particularly pretty, so if you let grow rogue somewhere, someone might consider it a weed, but it is also very valuable to sell, so as of right now, I find that unlikely.

Weeds are actually plants you don't want.

If they are not desired in your rose garden they are a weed.

If you're growing it to sell, it's not a weed. If it invaded your prize peony garden, it is.

pretty much

it grows almost anywhere, no fertiliser or pesticides needed

it might be one

Yes. More specifically, hemp is a weed. Good cannabis needs tender loving care in order for it to turn out good. But hemp can grow anywhere and it spreads like a weed. It’s such a tough plant that it takes special herbicides to kill it. Like a chemical called paraquat.