La-di-da (also written lah-di-dah or la-de-da) is a mocking way to describe someone who talks and acts as if they are finer, richer, or more cultured than everyone else. You say it when the posh voice feels put on: the long vowels, the little airs, the “look at me”. It is informal, a bit old-fashioned, and usually unkind in a light way — closer to teasing than to a serious insult, though it can still land. British English uses it a lot; American English knows it too. It does not have a tight dictionary meaning beyond that tone: it is the sound of fake poshness, copied so you can laugh at it. Though actually, people sometimes use it playfully about themselves, putting on a silly voice on purpose. It is never a compliment when you aim it at someone else.