"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

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