Definition from Wiktionary, the free dictionary
The adjective usage is not common in the US.
- (US) not a care in the world (used in a sing-song voice, for childhood rhymes)
- (US) Frequently heard in New Orleans, uttered by revelers in resorts of drink and music (used in a mildly pejorative sense, dismissing, for example, a man dressed in yellow bicycling Spandex with a silly walk, as Well, lah-di-dah. Also a term interjected when politicians waffle. Hmm. Sounds pretty lah-di-dah to me!)