Recently, reading Swift’s Gulliver’s Travels for the first time, I came to the nutty, impractical schemes pursued at the grand Academy of Lagado.

The first Man I saw … had been Eight Years upon a Project for extracting Sun-Beams out of Cucumbers, which were to be put into Vials hermetically sealed, and let out to warm the Air in raw inclement Summers. He told me he did not doubt in Eight Years more he should be able to supply the Governor's Gardens with Sun-shine at a reasonable Rate …

Now as many of you know, I’m a big fan of Gilbert and Sullivan. I couldn’t but jump, recalling Gilbert’s three young men making fun of “a Woman's college” (“maddest folly going!”) and the nutty, impractical schemes researched there. Such as:

And weasels at their slumbers
They trepan (they trepan);
To get sunbeams from cucumbers
They've a plan (they've a plan).
They've a firmly rooted notion
They can cross the Polar Ocean,
And they'll find Perpetual Motion,
If they can (if they can).

My first thought was that Gilbert had copied from Swift. But on reflection, I suspect that “trying to get sunbeams from cucumbers” was a proverbial saying (meaning “attempting the impossible”) that each author used.

Is it?

A quick perusal of offerings from a google search of the phrase leads me to conclude that in popular usage it refers to "flights of fancy" more than attempting the impossible. I detected what I would call a sarcastic or condescending tone to most of the applications of the phrase.

Lest these be seen as two of the same with different descriptions, I can see where the phrase might be applied to an attempt to prove the plausibility of astrology, but not to an experiment by physicists whose hoped for outcome was travel back in time.