So what the hell is a “shrub"? In a non-landscaping sense, it’s a method of preserving fruits and fruit juices that goes back hundreds of years. Martha Washington made shrubs; they appear in the works of Dickens. There are myriad recipes and techniques, but all involve mixing fruit with vinegar and sugar, letting things steep, then straining out the solids.
In the hands of a skilled Shrubber, the result is an intensely flavored concentrate, which is then mixed with a drinker’s fluid of choice, with delicious results.