Green sauce is a family of cold, uncooked sauces based on herbs, including the Italian salsa verde, the French sauce verte, the Spanish salsa verde, the German Grüne Soße or Frankfurter Grie Soß (Frankfurt dialect), and the Argentinian chimichurri.
The Mexican salsa verde, though also called a "green sauce" is instead based on tomatillos.
The basic recipe is probably from the Near East and, as such, is probably at least 2,000 years old. Roman legionaries brought it to Italy, from where it was exported to France and Germany. Evidence suggests that it was introduced in Kassel by the Italian trading families Bolongaro and Crevenna around 1700.
A possible origin of the German variant are French Protestant immigrants emigrating to Kurhessen in the 18th century. The German variant uses a different mix of herbs, since Mediterranean herbs were not available in Germany at the time.
The Italian salsa verde is a cold rustic sauce, and includes parsley, vinegar, capers, garlic, onion, anchovies, olive oil, and possibly mustard. The ingredients are coarsely chopped, formerly by hand, now often in a food processor. In some regions, cubed bread is soaked in vinegar and blended with the other ingredients, which creates an emulsion somewhat similar to a vinaigrette. In other regions, there is no bread.