Green’s Dictionary of Slang

(mainly UK juv.) the vagina.

1833		Satirist (London)27 Jan. 454/2: ‘What did the king say when he picked up that harter [...] Why he said he ony saw ki maly, which is the Gaelic for ‘the females’ —pence’’.
1890–1904		Farmer & Henley Sl. and Its Analogues V 45/2 Ninepence, subs. (common). — The female pudendum [An echo, on a liberal display of leg or underclothing, of the old alliterative retort, ‘Up to the Knees and ninepence’].