Can weed kill a ferret

Yes. I am taking an educated guess that you are talking about Cannabis sativa and not the crabgrass growing in your yard. Cannabis is a stimulate that increases the heart rate and metabolism of an animal. Being very small, ferrets already have a pulse of 225 beats per minute and a high metabolism that allows for a meal to be digested in four to six hours. Increasing these natural rhythms is extremely dangerous and could very well be fatal.