How did marijuana get the nickname "Mary Jane"?

-Mari(mary)juana(jane)Probably because Juan is a Spanish variant of John and since Juana is feminine, it became Jane (Jane is just a feminine version of John). And Mari just sounds like Mary.
Break up the name marijuana into mari & juana and you have MJ. MJ has been short for the girl's name of Mary Jane a long time. Check Spider-man comics.
it comes directly from the word marijuana.



mari/mary

juana/jane
MariJuana = MJ



Then people came up with names for MJ and I guess Mary Jane stuck.
in spanish, maria= mary

juana=jane