Plum blossom : nm A plum tree is a hardy tree cultivated for its edible fruits, plums.
Shake someone like a plum tree, very vigorously: scold him sharply (see below).
The expression “to shake like a plum tree”: to shake strongly, to rebuke severely, to reproach sharply.
If the meaning of “shake strongly” is self-evident, that of “rebuff” is a little less obvious. But that's just the figurative sense, considering that someone who's been given a harsh reprimand is completely pissed off. Without forgetting that in the XNUMXth century, the verb had the meaning of “roughing up”.
But why the plum tree? It probably and simply comes from former figured jobs of plums which since the XNUMXth century meant a blow or a wound.
Shaking plums is another different form of expression in which the tree is replaced by its fruit.
Note that shaking the plums can also mean "getting out of lethargy, of apathy to finally activate something". Or in other words to self-shake.