An improbable and visionary hope, ideal, scheme, etc, such as an opium smoker might have: He has some ambitious plans, mostly pipe dreams(1896+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with pipe-dream
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.