Jokes0 min ago
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The meaning is something which is dead, with no chance of revivial, in modern terminology, it refers to an idea or concept which is seen as beyond redemption. It probably comes from the war, when any sailors who died in the sea were left floating on the surface - dead in the water - which is a pretty final way of describing their status.
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Thanks for correcting my erroneous answer. As for 'the doldrums' - it's an rea of ocean (not sure where) which is famous for its lack of wind. Sailing ships could remain becalmed (or indeed dead in the water) for weeks at a time, a miserable experience, hence the adaptation of the phrase for anyone who is feeling a bit miserable.