dreamworld; dreamland; la-la land
Refers to an idealized, fantasy-like place or state of mind, often used figuratively for unrealistic or escapist thinking.
He's always like someone living in a dreamworld.
That plan is la-la land talk; it's not realistic.
Compound of 夢 (yume, 'dream') + の (no, possessive particle) + 国 (kuni, 'country'). Literally 'country of dreams'.