Jack Straw’s Castle, n. — Green’s Dictionary of Slang

Jack Straw’s Castle n.

[jackstraw n.; the common sl. image of the vagina as a trifle, a nothing]

the vagina.

1837		Comic Almanack Apr. 88: On his arrival, the wooden guns at Jack Straw’s Castle will be fired, and the town illuminated with moonshine.
1890–1904		Farmer & Henley Sl. and Its Analogues.