Agresto, or verjus in France, is the semi-fermented and reduced juice of unripe grapes. Centuries ago it was a very popular ingredient among both nobles and peasants, a sort of kinder, gentler version of vinegar, used in many of the same ways, but also in desserts. It's been out of fashion for a century or two, but it is still made locally, especially in Tuscany and Lombardy
In recent times, agresto has come to mean a sort of sauce to accompany meats. It's a mash of unripe grapes, finely ground nuts (walnutand/or almonds), onion, garlic, parsley, bread crumbs, sugar and sometimes other ingredients. Here's a recipe.