be in cloud-cuckoo land

To believe in or be absorbed by unrealistic, idealized, or fanciful ideas that are beyond the realms of possibility. If Tom thinks he'll be able to live off his bad poetry, he's in cloud-cuckoo land! OK, you're in cloud-cuckoo land if you think these harebrained schemes will actually work! Are you in cloud-cuckoo land? We'll never be able to convince a behemoth like Shmapple to invest in our tiny tech company!