Barley-water

Thin barley soup. One recipe of the day called for two ounces of pearl barley boiled in a quart of water. After that it was strained and a small piece of lemon peel added. Knight (177) says it was used to soothe fevers. It was also credited "as a medicament in the treatment of diarrhea in infants" (250). The OED (229) says it is "a drink made by the decoction of pearl barley; used as a demulcent." (Decoction: an extract from boiling down. Demulcent: a soothing substance.) Don't mistake it for the modern concoction, a fruit cordial of stronger constituency.

Context: 
and give them tea and barley-water
Opera: 
Act: 
Act II