('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]
(..) lt's been horrible. Well, you're safe now. We're all castoffs here. We been dumped, donated, yard-saled, secondhanded and just plain thrown out. But just you wait, you'll find being donated was the best thing that ever happened to ya. Mr. Lotso, do toys here get played with every day? (..)2010 Toy Story 3