Meaning: to trash or rid of something
Deep six refers to to six fathoms, or with a fathom being six feet, 36 feet. The origins of the phrase are obscure at best. It is currently thought that 36 feet was the rule of thumb needed for burial at sea to prevent a body from washing ashore. Of course, modern sea burials usually require a 100 ft minimum. The earliest documented usage in 1929 uses deep six as slang for grave.