“It’s right hard to say,” she said. “Suppose you and Scout talked colored-folks’ talk at home it’d be out of place, wouldn’t it? Now what if I talked white-folks’ talk at church, and with my neighbors? They’d think I was puttin’ on airs to beat Moses.”
“But Cal, you know better,” I said.
“It’s not necessary to tell all you know. It’s not ladylike – in the second place, folks don’t like to have someone around knowin’ more than they do. It aggravates ’em. You’re not gonna change any of them by talkin’ right, they’ve got to want to learn themselves, and when they don’t want to learn there’s nothing you can do but keep your mouth shut or talk their language.”

– Harper Lee

To Kill a Mockingbird, Chapter 12. The Finch’s black cook Calpurnia is trying to tell the children that you have to speak the language that people understand and you can’t go round acting like you are better than anyone else. An educated woman, she speaks proper English in the Finch home, but when she is with the black people in her community and at her church she speaks the way they do. Here, she is telling Scout what it means to be ladylike.