La La Land emerges as a slang term in the 1960s. It apparently comes from la-la as an imitation for the sound of someone who’s “crazy,” and land imagines a fictional, far-off place one goes to in such a state.

We also see la la as nonsense words used to drown out a person saying something someone doesn’t want to hear, associated with the gesture of plugging the ears with the fingers, thereby blocking out reality. This la la may have influence lalaland.

La La Land started being used in the late 1970s and early 80s as a nickname for Los Angeles, California, specifically to refer to Hollywood.  It puns on the city’s common abbreviation as LA and comments on the cinematic illusions made by its film industry—as well as the enchantment of fame and the “out of touch” lives of its celebrities.

Paris, France was called Lalaland in the 1920s, based on the French-sounding exclamation of excitement ooh la la, but this earlier use appears to have little bearing on the later nickname for LA.

La La Land came into prominence with the 2016 award-winning musical romantic-comedy film La La Land.  It follows the romance of struggling artists Sebastian (Ryan Gosling) and Mia (Emma Stone) as they try to make it in Hollywood. The film’s pursuit of dreams, blissful romance, and LA setting inspired its title. It gained even more notoriety due to the 2017 Oscars snafu, where La La Land was announced as best picture when it was actually Moonlight.

Source: Dictionary