Well, before cannabis was made illegal and called 'marijuana', it provided a useful commodity called hemp. Hemp could be used for most things back in the 14th century, and it often was. The first canvas' were actually made from hemp, which comes from cannabis! Thus, it was called a 'canvas' because of the relation it had to the cannabis plant!
It's weird that words work like that. If you have an interest in the origin of words, you should consider studying Etymology, or just clicking the source to take you to a website where you can find out about the history of certain words!
