Green’s Dictionary of Slang

melting pot n.

the vagina.

1890–1904		Farmer & Henley Sl. and Its Analogues.
1896		Farmer Vocabula Amatoria (1966) 85: Creuset, m. The female pudendum; ‘the melting-pot’.
1980		Maledicta IV:2 (Winter) 182: Occasionally, the colloquialism acknowledges the functions of the part, as is seen with [...] baking pot (where you keep a bun in the oven).