Barley water, usually flavoured with lemon or other fruit, is a traditional British herbal tea. It is made by boiling washed pearl barley, straining, then pouring the hot water over the rind and/or pulp of the fruit, and adding fruit juice and sugar to taste. The rind may also be boiled with the barley.
Drinking boiled grain in water, strained or not, is an ancient practice. Kykeon (Gr. κυκεών, from κυκάω, "to stir, to mix") was an Ancient Greek drink made mainly of water, barley and naturally occurring substances. It was used at the climax of the Eleusinian Mysteries to break a sacred fast, but it was also a favourite drink of Greek peasants. It is also a popular drink among Punjabi peasants. It is called 'sattu' in Punjabi.
A Scottish example is atholl brose which is made with oats. In Mexico grain-based aguas frescas are made by street vendors using similar methods. Roasted barley tea is a popular traditional Asian equivalent. Barley water is a very common drink in South Korea.