more often known as Mary Janes, since wearing just one can be a bit tiring, and lead to joint disorders in later life.
These are those super-cute, usually shiny, usually black shoes with a rounded, closed toe and a thin strap that buckles over the instep — school girl shoes. They come in other colors as well, but black is pretty much considered the classic for the style.
The slang term for cannabis or marijuana is one I have only ever heard used by cops or in those annoying PSAs. Perhaps, when not used to refer to shoes, this term is a good warning sign that somebody is trying to set you up? If you run into someone who offers you Mary Jane, and they aren't holding a shoebox containing ruby slippers or something similar, walk away.