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.

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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

To deep-six means to get rid of, to ignore, or to bury something. It comes from the six feet depth of a grave.

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.

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Blowing a load inside a pussy.