Definition of deep sixing:
(v) :
(informal, idiomatic, transitive) To discard, get rid of, or cancel; to completely put an end to something.
(n) :
(informal, idiomatic) An ejection, discardment or destruction.
(n) :
(informal, idiomatic) The grave or death; also, the (notional) place where something is discarded.
(v) :
(informal, idiomatic, transitive, uncommon) To submerge (something) in water.