I felt I should add to AgentIpso's w/u and add the etymology of the phrase "pipe dream".

This phrase appears to have been coined in 1896, although there are conflicting references that claim that it first appeared in written form in the book "Love Sonnets of a Hoodlum" by Wallace Irwin in 1901.

The phrase is throw back to the day when opium was a legal drug. It refers to the mind altering effects of the drug and the flights of fantasy and hallucinations that resulted from smoking the drug in an opium pipe. Thus "Pipe Dream".