blue-sky

1. Unrealistic and impractical: “[The author] shows what is testable physics, what is philosphy's domain, and what is blue-sky nonsense” (Ann Finkbeiner).
2. Having a cloudless sky; clear: a blue-sky day.

To hold or express unrealistic or impractical views, especially in estimating something. Often used with it.

The noun has one meaning:

Meaning #1: the sky as viewed during daylight
  Synonyms: blue, blue air, wild blue yonder