It's often used to describe someone who is:
* Overly concerned with social graces and etiquette.
* Unrealistic or out of touch with reality.
* Putting on airs to appear sophisticated.
The phrase suggests a person who is superficial and excessively concerned with appearances, rather than substance.
Origin: The phrase is a play on the words "la-di-da," a similar expression used to indicate affected snobbery. It likely originated in the late 19th or early 20th century, but its exact origins are unclear.
Example:
* "She treated everyone with a lah de dah attitude, as if she was above them."
* "The lah de dah restaurant served tiny portions of food that were overpriced."