Aboard a ship, on the ocean, as in Within a few hours the ship would be out at sea. During World War II a famous American newscaster addressed his radio broadcasts to listeners everywhere, including "all the ships at sea." [1300s]
Also, all at sea. Perplexed, bewildered, as in She was all at sea in these new surroundings. This idiom transfers the condition of a vessel that has lost its bearings to the human mind. Charles Dickens used it in Little Dorrit (1855): "Mrs. Tickit ... was so plainly at sea on this part of the case." [Second half of 1700s]
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