The expression “full of prunes” means, peppy, lively, energetic, in high spirits, feeling one’s oats, rarin’ to go.
“Beans” was the first, and was originally said of horses after a feeding of beans raised for fodder, “horse beans,” so called.
Undoubtedly the spirited state of a bean-fed horse was observed in remote times, Romans also used beans as fodder, but I find nothing equivalent to the current expression before its own rise less than a hundred years ago.
“Full of beans” is now also used slangily to mean foolish or silly, possibly because a person in high spirits often permits his superabundant energy to express itself ridiculously. The substitution of “prunes” came into use at least seventy years ago, but a satisfactory reason for it is difficult to determine.
Perhaps the dietary effect of an overindulgence in this comestible may have had something to do with it.
We are reminded, at least, of a mother who urged her son, training for scholastic track events: “Eat plenty of prunes, Jimmy; they’ll make you run.”