Etymology From pipe + dream, referring to the fantasies experienced when smoking an opium pipe. Compare Old English pīpdrēam (“the sound or music of a pipe”), which is formed identically but has a different meaning.
1
Idiomatic A desire or idea which is unlikely to materialize, or a plan which is unlikely to work; a near impossibility.
I think that his plan to become a professional athlete is a pipe dream and that he should stay in school.
Only a year ago it would have needed a "super-Micawber" to be optimistic that the railways would once again pay their way. But it was no longer a pipe dream that B.R. could make a profit, the way to do it was now clear.1962 December, “Dr. Beeching Previews the Plan for British Railways”, in Modern Railways, Shepperton, Surrey: Ian Allan Publishing, →ISSN, →OCLC, page 376:
To have a desire or idea which is unlikely to materialize, or to imagine a plan which is unlikely to work; to dream of a near impossibility.
Love is life sublime, / Ev'ry moment that I'm / Only wasting my time / Pipe dreaming of you, / Pipe dreaming of you, […]1946 April, Cole Porter, “Around the World in Eighty Days [Pipe Dreaming]”, in Robert Kimball, editor, The Complete Lyrics of Cole Porter, New York, N.Y.: Da Capo Press, published 1992, →ISBN, page 378, column 1: