Pipe dream

Here's an interesting entry from the book Why You Say It by Webb Garrison

Pipe Dream
Should you come up with a really novel idea or plan, chances are that those who first hear of it will pooh-pooh it as a "pipe dream."
In its earliest decades, the pipe dream was so far out that it seldom made contact with reality. That's because it was produced by opium, brought to England and Europe by merchants who penetrated the Orient and began peddling the stuff yielded by some kinds of poppies.
Pipe dreams influenced several notable literary figures, with Samuel T. Coleridge being high on the list. But by and large, fantasies produced by opium were that and nothing more.