“Is this a weed?” my father used to ask my mother when I was small, having already pulled up the apparently offending piece of organic matter from the ground.
Some of the time, my mother wanted it there and was annoyed. Mostly, however, she was just annoyed that he’d pulled it out first. If she had wanted it, it was too late now!
The ‘evil weed’ is a not-so-secret term for cannabis, perhaps a reference to the fact it is illegal and could have various affects on you. The urban dictionary might define the term as a physically weak person, used as an insult particularly among children, based on a perception of a plant that is stringy, small and useless.