It has been my understanding that a shrub is a small to medium sized bush, several of which together can make a shrubbery. Here is a cranberry shrub:
This is also, apparently, a cranberry shrub:
To the right, are two bottles of grape shrub. Which grows on a vine rather than on a shrub.
Looking for ways to use my grapes and cranberries, I came across recipes for ‘shrub’ – a kind of cordial made with sugar and vinegar or alcohol (I chose vinegar) and which will keep in the fridge for several months. The making of it uses lots of utensils and pots for a rather small result. After steeping overnight, the cooked fruit is sieved which requires patient effort. It looks like the kind of drink which might be useful in winter as an immunity boost, diluted and served hot or cold.
Anne-O-tations 
I had no idea a ‘shrub’ could be a drink. I’ll be interested to learn how it turned out. — Miraz
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One source claims it originated with smugglers who used it to disguise the salty flavour of sea-damaged brandy.
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The Shorter Oxford suggests another meaning of shrub is to scratch the body or fidget. An immunity boost might help to avoid this.
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That’s interesting. My old Collins dictionary had only the plant shrub and it occurred to me that your Shorter Oxford might have other meanings.
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