Buttermilk is a liquid that remains after making butter. It tastes a little sour and is often used in cooking and baking, especially in recipes like pancakes, biscuits, and marinades. Full definition
On low speed add the flour mixture in thirds alternating with buttermilk in 2 equal additions, beating until smooth after each addition and scraping down the sides of the bowl a few times. (cinnamonspiceandeverythingnice.com)
They would scoop it all into a ball and soak it in buttermilk for an hour. (sbnation.com)