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.