1
general::
1. 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] 2. 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."
American Heritage Idioms
2
general::
Phrase(s): deep-six someone or something
Sl. to get rid of someone or something; to dispose of someone or something. (Refers originally to burying someone or something six feet deep, the standard depth of a grave.) • Take this horrible food out and deep-six it. • That guy is a pain. Deep-six him so the cops will never find him.
McGrawhill's American Idioms And Phrasal Verbs