Green’s Dictionary of Slang

shady spring n.

also Castalian spring

the vagina.

[UK]Bridges Burlesque Homer (3rd edn) 62: [She] hated paris in her heart, / Because he’d seen her shady spring, / And did not think it was the thing.
[UK]Harris’s List of Covent-Garden Ladies 24: All who bathe in her Castalian spring will be overwhelmed witha flood of delight.
[UK]Farmer & Henley Sl. and Its Analogues.
[[US]Joe Louis Walker ‘Shade Tree Mechanic’ [lyrics] Cause I’m your shade tree mechanic / And heaven knows that I’m the one / I can oil and lube you / And not lose a single drop, no no / I can cool you down / When you start to runnin’ hot / I’m your shade tree mechanic].