Verjuice actually means "green juice." It is an acidic liquid made from unripe sour grapes (or crabapples). It was used in Roman cookery, but was very popular in Medieval cooking, and even used into the 19th century. It is very popular in Iranian cuisine and is starting to be given more attention in modern American cookery. It was also used historically as a medicine.
Verjuice is a good substitute for lemon juice or vinegar. It has a sour taste but fruity and not quite as sour as vinegar. It's mostly found in recipes for salad dressing, soups, stews, and sauces.
Mutton with Yellow Sauce
The Good Wife's Guide, Le Menagier de Paris, A Medieval Household Book. Translated, with Critical Introduction, by Gina L. Greco & Christine M. Rose, Cornell University Press, 2009.
Cut the meat--it must be flank--into pieces when completely raw. Cook it in water, then grind a piece of ginger and some saffron and thin it with verjuice, wine, and vinegar.
by Sussannah Carter. New York: 1803.