Green’s Dictionary of Slang

turnpike n.

the vagina.

1660		Wandring Whore III 3: Disclose thy curled, silver-colour’d turn-pikes and perfum’d Cabinet lyes in thy vally sweeter than Violets.
1743		‘Roger Pheuquewell’ Description of Roads to Merryland 2: I have ventured to substitute another Word in its Room [...] calling them Turnpikes.
1890–1904		Farmer & Henley Sl. and Its Analogues.