The grave or death; also, the (notional) place where something is discarded.
Attested since the 1920s as a noun, most likely from the nautical cry "by the deep six" indicating a depth of 6 fathoms (36 feet, 10.97 metres) (a depth at which something thrown overboard would be difficult to recover), though possibly also a reference to the common depth of graves (six feet).
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