A conjunction meaning 'and yet' / 'despite that', essentially equivalent to なのに; often used in colloquial speech as a less formal variant.
天気予報は晴れだのに、朝から雨が降っている。
The forecast said it would be sunny, and yet it's been raining since morning.
彼はお金持ちだのに、質素な生活をしている。
He's wealthy, yet he lives a frugal life.
忙しいだのにと言い訳ばかりして、手伝おうとしない人がいる。
There are people who just keep making excuses like 'I'm busy' and won't try to help.