Green’s Dictionary of Slang

fan n.2

[fanny n.1 (1)]

the vagina.

[UK]‘I Wanted — I Could Not Tell What’ in Fal-Lal Songster in Spedding & Watt (eds) Bawdy Songbooks (2011) III 6: My daddy he bought me a knot, — / With a fan and a new fashioned fly.
[UK] ‘He’ll No More Grind Again’ in Ri-tum Ti-tum Songster 30: Red-haired Moll, his sole delight, / Chief blowen of the train, / Rubs up her precious Fan., and cries, / ‘Poor Bill will never grind you again’.
[US]Maledicta IV:2 (Winter) 183: Then there are the euphemisms used in more respectable literature [...] what were presumably originally pet names such as her fan (short for fanny probably the commonest semi-polite term for the tenuc [backslang] in the U.K. In the U.S.A. this word more commonly refers to the posteriors).