Definition from Wiktionary, the free dictionary
Possibly from Los Angeles.
- A state of mind characterized by a frivolous or unrealistic lack of seriousness.
1958, TV Guide, volume 35, number 36–39:
- ... back to her new home in Calabassas, near Los Angeles. Her years in Phoenix, where she started her own zoo, ended in divorce, "and now I'm back in La La Land," ...