a large British passenger ship which was people considered impossible to sink, but which hit an iceberg in the Atlantic Ocean on its first journey in 1912, and as a result sank, killing more than 1500 of its passengers. There have been many books and films about this event. People sometimes use the phrase "like re-arranging the deck chairs on the Titanic" to describe the actions of a person or organization which makes small and useless changes to a situation, when the situation is already hopeless and certain to fail.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.
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