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Phrase(s): a pipe dream
Fig. a wish or an idea that is impossible to achieve or carry out. (From the dreams or visions induced by the smoking of an opium pipe.) • Going to the West Indies is a pipe dream. We’ll never have enough money. • Your hopes of winning a lot of money are just a silly pipe dream.
McGrawhill's American Idioms And Phrasal Verbs
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A fantastic notion or vain hope, as in I'd love to have one home in the mountains and another at the seashore, but that's just a pipe dream. Alluding to the fantasies induced by smoking an opium pipe, this term has been used more loosely since the late 1800s.
American Heritage Idioms