An ordinary fruit juice from Earth that Guinan offered to Worf in Ten Forward, and which he declared a warrior's drink and adopted as his regular order.
Prune juice enters Star Trek in the episode Yesterday's Enterprise. In the opening scene Guinan remarks that Worf always drinks alone and never tries anything new, and hands him a glass of an Earth drink. He tastes it and approves at once.
From then on it is his usual drink, ordered in Ten Forward and elsewhere, generally chilled.
When he arrives on Deep Space Nine in The Way of the Warrior he orders it at Quark's bar, to the Ferengi's evident surprise, and it remains his drink for the rest of that series. The joke rests on the contrast between a fierce Klingon officer and a drink humans think of as mild and medicinal.