To hide or dispose of something in such a way that it will be extremely difficult or impossible to find or recover, even for the person doing the hiding. Get rid of. Nautical in origin, it comes from a call indicating a depth of six fathoms (over ten meters), a depth from which recovery of an item tossed overboard would be very difficult.

to stow something away in place where it will be impossible to find. the six refers to six fathoms (or 36 feet), the minimum legal depth for a burial at sea

The six refers to a grave, as in six feet under.
The deep refers to the ocean.

Together we have a death at sea, where youre grave is much futher down than 6 ft. Today we just use it to say "throw away" or something like that.

What a Japenese man dreams he could give a woman

death