Green’s Dictionary of Slang

1. the anus.

1664		C. Cotton Virgil Travestie (1765) Bk I 13: From th’ Muzzle of his double Jug, / The Winds burst out with such a Rattle / As he had broke the Strings that twattle. / Bounce, cries the port-hole, out they fly.
1984		Partridge DSUE (8th edn) 913/1: from ca. 1660; ob. by 1930.

2. the vagina.

1800		Nautical observations on female dress [cartoon text] ‘Women are like Crazy Hulks in a rough Sea, the Port holes are Hardly Secure!!!’ ‘Or like Great Guns, your honor! Long & lank with barely an apron to cover the touch hole’.
1834		‘Randy Mots of London’ in Libertine’s Songster in Spedding & Watt (eds) I 139: The jolly tar so full of glee, / Admires the tempting open C—, / And ever dearly likes to be, / In a port-hole where there’s fun done.
1984		Partridge DSUE (8th edn) 913/1: from ca. 1660; ob. by 1930.