Green’s Dictionary of Slang

mary jane n.1

also mary
[generic use of female name]

1. the vagina.

[UK]Farmer & Henley Sl. and Its Analogues.
[US] in P. Smith Letter from My Father (1978) 76: For years I played with Lydia’s ‘mary’.

2. (US black) a skirt with a slit up one side; also attrib.

[US]P. Oliver Blues Fell this Morning 99: The legs of the passing brownskin girl with the side-slit ‘Mary Jane’ skirt.

3. (US) a lesbian.

J. Grey Twilight Girls n.p.: Your little Edie is a Mary Jane — a chicken for some dyke.
[UK]J. Morton Lowspeak 97: Mary – a lesbian.