I got "cloud cuckoo land" from the Germans, who use the term as a catch-all for crazy, impractical ideas. "Wolken Kuckucks Heim," or thereabouts, check that spelling! It turns out that it goes back much further than that.
Generally I don't spend much time reading complex and difficult literature, but I do like to read ABOUT those things. Summaries, criticisms, book reviews, those I have the time and the patience for. So I'm reading this Signet Classic edition of "Books That Changed The World," by Robert Downs. Good stuff, most of which is new to me.
I'm still in the Greeks, and the "cloud cuckoo land" thing comes from a play by Aristophanes (448 - 380 BC), "The Birds." It was written at a difficult period for Athens, lots of war and discontent going on. Nephelococcygia is a city in the clouds, run by birds, and very peaceful, evidently. Comedy results.
Not going to look it up, not me, but I enjoyed reading about it.