Green’s Dictionary of Slang

a coy ref. to the vagina.

[1787		Harris’s List of Covent-Garden Ladies 19: The titillation of nature in her favourite spot below, feelingly calls for the Priapian weapon to receive it in her sheath at its most powerful thrust up to the hilt].
1837–8		‘The Mouthful’ in Cuckold’s Nest 14: To tell you, sweet Kate, I am not ashamed, / When you fell, I saw you’d got a mouth down below, / And if you’d let me fill it, you’d banish my woe.
1979		E. Torres After Hours 87: She wasn’t red all over, but she sure had fire down below.
1994		(con. 1930s) K.C. Kearns Dublin Tenement Life 143: I had an ointment that was famous all over the city for women with an itch, as they say, ‘down below’.
2000		Indep. Rev. 12 Jan. 8: They use a jolly vocabulary of ‘tummies’ and ‘down below’.
2020		(con. 1991-94) W. Boyle City of Margins 174: ‘But down below was a mystery. A lot of early schoolyard talks [...] revolved around speculating on women’s private parts’.