cloud cuckoo land

Từ điển Oxford Advanced Learner 8th

ˌcloud ˈcuckoo land BrE NAmE (BrE) (NAmE cloud·land, ˈla-la land) noun uncountable (informal, disapproving)
if you say that sb is living in cloud cuckoo land, you mean that they do not understand what a situation is really like, but think it is much better than it is
See also: ↑la-la land

Word Origin:
[cloud cuckoo land] late 19th cent.: translation of Greek Nephelokokkugia, the name of the city built by the birds in Aristophanes' comedy Birds, from nephelē ‘cloud’ + kokkux ‘cuckoo’.