noun

• (literally, military) A destructive capacity that exceeds that needed to destroy an enemy; especially with nuclear weapons.
  "[The] increase in nuclear missiles available to the West was operationally unnecessary and would only add to the existing nuclear overkill; mixed manning was a formula for military disaster; […]"
• (literally) Destruction beyond what is necessary to kill, especially in murder.
  "Bodies that have been stabbed or shot multiple times—far beyond what was sufficient to kill the victim (overkill)— suggest that the victim and assailant knew each other, […]"
• (by extension) An unnecessary excess of whatever is needed to achieve a goal.
  "24 hours of TV coverage of the US election verged on overkill."
• (manufacturing) An unnecessary excess of disposal because of too high criteria of inspection.

verb

• (transitive) To destroy something with more (nuclear) force than is required.
• (manufacturing) To dispose of too many items because of too-high criteria of inspection.