Green’s Dictionary of Slang

the female perineum.

[1884		‘Experiences of a Cunt Philosopher’ in Randiana 66: Cunt [...] /A nasty, slimy, slobbery slit, / Half-an-inch betwen arse and it; / If the bridge were to break t’woul’d be covered with — ].
1997		Roger’s Profanisaurus in Viz 87 Dec. n.p.: Niffkin’s bridge n. Female equivalent of the Biffin bridge or barse (qv).