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pipe dream   (pipe dreams plural ), pipe-dream A pipe dream is a hope or plan that you have which you know will never really happen.  n-count 
You could waste your whole life on a pipe-dream.  

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"Collins Cobuild English Dictionary for Advanced Learners 4th edition published in 2003 © HarperCollins Publishers 1987, 1995, 2001, 2003 and Collins A-Z Thesaurus 1st edition first published in 1995 © HarperCollins Publishers 1995"