Green’s Dictionary of Slang

cloister n.

the vagina.

c.1670		Rochester ‘A Dream’ n.p.: I look’d and saw the blind boy’s happy Cloyster/ Arch’d on both sides, lay gaping like an Oyster.
1783		Harris’s List of Covent Garden Ladies 24: [T]wo living diamonds [...] forcibly invite the Blind Boy to the happy cloyster.