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1.
Iron Coffin was a rueful term coined by German submarine ("U-boot" or "U-boat") crews to describe their vessels during World War II. This term reflected the submariners' awareness that submarines are usually sunk while they are submerged, so that the crew of a sunken submarine is interred forever in the hull at the bottom of the sea.

The term is descriptive and makes the desired point, although during the war submarines were made of steel, not iron.

The truth of the term is evident in the fact that between 28,000 and 39,000 U-boat men died in the War, representing a casualty rate between 75% and 90%. The lesser number is most often quoted. The wide difference in estimates may result from the fact that many U-boat men died in air attacks while ashore, or were killed after being re-assigned to Army units during the last months of the conflict.

In the early 1970s the term gained some familiarity among English speakers after the publication of a memoir of the same name written by Herbert Werner, a former U-boat commander.
When a submarine sinks, it becomes an iron coffin.
by Walking Point April 30, 2009