Green’s Dictionary of Slang

the vagina.

c.1495		Skelton Dyvers Balettys and Dyties Solacyous ii 26: It is perlous for a horseman to dyg in the trenche.
c.1616		Fletcher Loves Pilgrimage II iii: There be Trenches Fitter and warmer for your years, and safer Than where the bullet plaies.
1674		Cavendish Triumphant Widow II ii: By your Venus trench You should love a Wench.
1682		Southerne Loyal Brother II ii: I never fear an enemy when I have won his trenches.
1714		N. Ward Whole Pleasures of Matrimony 33: And if his Pleasure be so great in opening of the Trenches only, how much more must his Satisfaction be redoubled, when he gains the Fort, whether it be by Storm, or by Surrender.
1734		Love in all its Shapes 34: The Father-Guardian returned to the Assault, and finding the TRENCHES open, he soon made himself master of the Fort, with great vigour.
1768		Gentleman’s Bottle-Companion 8: He fill’d up the trench with a faggot of length, / And ramm’d in his charge with a ramrod of strength.
1772		Bridges Burlesque Homer (3rd edn) 35: The smooth rimm’d trenches / Of sooty, sweaty, Negro wenches.
1798		‘Medley’ in Hilaria 40: If wedlock’s your plan, ere you scheme to open trenches, / Humbugs pray take heed of our modern made-up wenches.
1896		Farmer Vocabula Amatoria (1966) 256: Tranchée, m. the female pudendum; ‘the trench.’.
1930		Razaf & Williams ‘My Man O’ War’ 🎵 I got myself a military man / [...] / He storms my trench and he’s not afraid / His bayonet makes me cry for aid.
1941		in G. Legman Limerick (1953) 263: The unlucky wench / Got it caught in her trench.
1980		Maledicta IV:2 (Winter) 182: The simplest words in common use for this ‘nasty thing’ [...] are those accepting the female sexual apparatus as a simple receptacle. These include [...] trench, gulley or gulley-hole.