I'm reading Fragile Things, a collection of short stories by Neil Gaiman. The first story, "A Study in Emerald," is a Sherlock Holmes tale. In it, Holmes, who is in disguise as a theatrical promoter, and an actor are discussing grand plans for a tour of America. They are smoking tobacco pipes and have this exchange:
"This is most exciting," said Vernet (the actor). "I hope it will not turn out to have been a pipe-dream!"
"No sir, it shall not!" said my firend (Holmes), puffing on his own pipe, chuckling at the man's joke.
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I'd wondered before why an unlikely-to-happen plan is called a pipe dream, and it never occurred to me that it was a reference to smoking pipes and not plumbing pipes. As soon as I read that, I realized it was probably a reference to opium, which a check of the American Heritage Dictionary confirmed. It defined pipe dream as "a fantastic notion or vain hope" and gave the etymology as "from the fantasies induced by smoking a pipe of opium."
I'd bet the OED has more information about this, but of course, I CAN'T CHECK. Argh. (UPDATE: A friend checked the OED for me on this, and I was wrong, there is no further etymology.) Merriam-Webster Online does give a date of 1896 for the coining of pipe dream, squarely in the days of Sherlock Holmes. (UPDATE: The OED dates it to 1890.)