The phrase 'cool water sandwich' was popularized in mainstream culture by the Blues Brothers Jake and Elwood (Jon Belushi and Dan Akroyd). In their song Rubber Biscuit, Elwood tells us that:

The other day I had me a cool water sandwich and a sunday-go-to-meetin' bun.
That is to say, I had me a slice of watermelon and took a little lady to church, bow bow bow.

So there you have it. Cool water sandwich = slice of watermelon.

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