Green’s Dictionary of Slang

a derog. excl. implying the recipient is pretentious or snobbish.

1960		(con. 1930) J. Havoc Early Havoc 203: ‘Patsy [...] is as close to being a gentleman as anybody I know.’ Scotty laughed out loud. ‘La-dee-da!’ he hooted.
1999		J. Cameron Brown Bread in Wengen [ebook] ‘Miss Mannion,’ went Rupert. ‘Oh lah-di-dah,’ goes Noreen.
2000		W.T. Vollmann Royal Family 543: Well, la-di-da.