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Origin likely
“Deep six”
What it means
To dispose of something permanently.
Where it comes from
A fathom is six feet, the span of a sailor’s outstretched arms, and it was the unit of every depth sounding taken at sea. Six fathoms was reckoned deep enough that anything committed to the water would not come back.
How sure are we?
The nautical origin is well supported, though the phrase also draws on the traditional six-foot depth of a grave, and the two senses likely reinforced each other.