('US') All kinds of people attended the event, including politicians, celebrities, and 'just plain folks'. [=ordinary people]
The answer is (as) 'plain as day'. = The answer is 'as plain as the nose on your face'. [=the answer is very obvious]
She describes herself as a 'plain Jane'. [=a woman who is ordinary looking and not beautiful or glamorous]
(..) I have many wells flowing at many thousand barrels per day, so I like to think of myself as an oilman. As an oilman, I hope that you'll forgive just good old-fashioned plain speaking. Now, this work that we do is very much a family enterprise. I work side by side with my wonderful son, H. W. (..)2007 There Will Be Blood
(..) If we could take a few minutes, there's something I'd very much like to talk with you about. Mother, would you take the girls outside, please? I believe in plain speaking, so... This is a beautiful ranch. I love hunting for quail. My boy has been sick, you know. He needs fresh air. Doctors all say (..)2007 There Will Be Blood