|slang (US) (tr) to dispose of (something, such as documents) completely; destroy|
|[C20: from six feet deep, the traditional depth for a grave]|
To dispose of, discard, or get rid of: “The board of directors deep-sixed the proposal without even reading it.” This phrase is derived from the noun “deep six,” meaning burial at sea and referring to the depth of water necessary for such a burial. The term was later used as slang for a grave (customarily six feet underground) and, by extension, as a verb meaning “to kill.”
deep six definition
Also, give or get the deep six. Burial at sea. For example, When the torpedo hit our boat, I was sure we'd get the deep six. This expression alludes to the customary six-foot depth of most graves. [Early 1900s]
Disposal or rejection of something, as in They gave the new plan the deep six. This usage comes from nautical slang of the 1920s for tossing something overboard (to its watery grave; see def. 1). It was transferred to more general kinds of disposal in the 1940s and gave rise to the verb to deep-six, for "toss overboard" or "discard."