cloud-cuckoo-land

An idealized mythical domain, as in That idea about flying cars is straight out of cloud-cuckoo land. This expression originated as a translation from the Greek of Aristophanes' play The Birds, where it signifies the realm built by the birds to separate the gods from humankind. It came into use in the 1820s. During the 19th century it began to be used for a place of wildly fanciful dreams, unrealistic expectations, or the like, and it also acquired the connotation of “crazy” (from cuckoo, slang for “crazy” since about 1900). Also see la-la land; never-never land.