Sounds like a job for The Interpreter.
The word: Lyonnaise (lee-oh-nays).
Its origin: Lyon, France.
What it means: In the style of Lyon.
What it really means: A dish cooked with chopped, often caramelized onions. Or, when modifying the word “sauce,” it refers to a gravy comprised of white wine, vinegar and onions. So... think onions.
Where you’ve seen it: French restaurants, brasseries and the occasional grandmother’s house.
How it usually appears on menus: Potatoes lyonnaise; sauce lyonnaise; salad lyonnaise.
Fun fact for dinner conversation: “The invention of sauce lyonnaise is often attributed to Duke Philippe de Mornay, a French Protestant writer who served as Henry IV’s right-hand man from 1577 to 1582.”
Appropriate response: “Okay. Pass the potatoes.”