Actually the Spanish word for Mary is MarÃ*a. "Marijuana" is a shortened form of MarÃ*a Juana (Mary Jane).

And my Dictionary of American Slang also says "pot" comes from the Mexican word potiguaya, which refers to the cannabis leaves, so that's probably the right answer.

Isn't it funny how we adopted these obscure Mexican words for weed when most Mexicans never call it that? They usually just say hierba or mota or yesca.