Listing particle used in the pattern ～だの～だの to give non-exhaustive examples, often with a critical or dismissive tone. Similar to ～とか～とか but more casual and sometimes negative.
He keeps complaining about this and that, saying it's too expensive, too small, and so on.
He keeps making excuses like he has no money, he has no time, and so on.
あの店は高いだの、まずいだのと悪口を言われている。
That restaurant gets badmouthed as being expensive, tasting bad, and whatnot.
She's going on about being busy, being tired, and so on.
Derived from the copula だ combined with the particle の; the exact historical development is uncertain, but it functions as a colloquial listing particle.