The liquid of a dried Earth plum, a traditional treatment for constipation.
Guinan presented to Worf in 2366. Impressed, he christened it a "warrior's drink." Worf's parents later told Guinan they never served him prune juice because Worf never wanted anything but Klingon food. He still enjoys them on DS9, ordering at Quark's before going on the Defiant mission that led to his extradition hearing.