Real Talk, in which I explain why we say weird things like “plugged nickel” or “cats pajamas.” Also, a place where I can send my boyfriend when he asks the meaning of weird phrases like “plugged nickel” or “cat’s pajamas.”
Deep breath, kids. La-di-da is a corruption of lardy-dardy; “dardy” actually has no meaning, it’s tacked on only for rhyming purposes. This leaves us with “lardy,” itself a corruption of “lady” meaning fancy. Which ultimately gives us la-di-da, as in, “Oooh, aren’t you la-di-da, eating ice cream out of a bowl instead of digging your fingers into the carton!”
Sidenote: La-di-da had fallen into disuse until Annie Hall brought it back. Well, lardy-dardy!