"La-di-da" (and other spellings such as "lah-dee-dah" and even "lardy-dardy") is a
description that started out as a
mockery of the refined
English pronunciation of the
upper classes. (However, it is not limited to England in its use.)
As a verb, it means to use
affected manners or speak
pretentiously; as a noun, it implies that someone is acting more
genteel than they really are. It can also be used as an expression of
scorn, responding to someone's fancy plans with a sarcastic "La-di-da!" The
Oxford English Dictionary's citations range from
1861 to
1901, but the movie
Annie Hall's title character gave the expression a boost of popularity in
1975.
Sources:
http://dictionary.oed.com
http://wordsmith.org/awad/archives/0302
http://wordsmith.org/awad/awadmail74.html