the.com/nail in the coffin

means A decisive, final event that guarantees the failure or end of something already struggling.

from The image is literal: once coffin nails are hammered in, the lid is sealed for good — there's no reopening it. The phrase 'another nail in one's coffin' surfaces in English by the 18th century, often applied to habits like drinking or smoking that were said to hasten death one nail at a time. Over time it broadened from personal mortality to anything doomed — a business, a plan, an argument — each setback another nail driven into a lid that was already closing.