To be peppy, energetic or frisky; to be in high spirits, rarin’ to go, feeling your oats, enthusiastic.
This phrase has been around for about a hundred and fifty years, and it seems generally understood that it arose in horse racing. Horse beans have been raised for fodder at least since Roman days, and they provided the animal with more energy than plain grass. So a horse that was ‘full of beans’ was more likely to win a race than one that was not.
Here are some examples from the OED:
1870 Daily News 27 July 5 The horses […] looked fresh and beany.
1843 R. S. Surtees Handley Cross II. vii. 199 [Hounds, horses], and men, are in a glorious state of excitement! Full o’ beans and benevolence!
The substitution ‘full of prunes’ came into use at least seventy years ago. But the meaning was expanded to include: incorrect; full of shit; nonsensical, foolish, silly, and stupid. These meanings seem apt enough since prunes can cause gas, which is hot air, which also has the meaning ‘nonsense.’ ‘Full of beans’ can be used to mean foolish or silly, because a high-spirited person often lets his abundant energy express itself ridiculously.
Prunes are also used for their laxative effect, which would explain the ‘full of shit.’ Another 19th century use of “full of beans” applied to a person “whom sudden prosperity had made offensive and conceited.” That could also be seen in the sense of being “full of it.”
Aside from all these meanings attributed to ‘full of beans or prunes,’ we humans have had a long relationship with beans. For example, Pythagoras told his followers “not to love beans,” but that may have been a warning against meddling in politics, because beans were used as markers in political elections. So were pebbles, I believe.
A blog called Daily Writing Tips says that Egyptians buried them with their dead, and Homer mentioned them in the Iliad. On the ancient Roman feast called the Lemuria (or Lumuralia), the pater familias (father of the family) got out of bed at midnight to walk around the house barefoot, throwing black beans over his shoulder. The rite was intended to exorcise any malevolent spirits that had accumulated in the household during the previous year.
Perhaps I should walk around the table my computer sits on. A few black beans might chase away the malevolence brewing inside that instrument.
‘Feeling your oats’ probably also arose from horse racing. Oats, like horse beans, would provide a horse with abundant energy. It was copied, in a sense, with the advertising slogan, ’He’s feeling his Cheerios,’ used for breakfast cereal in the early 1950s.
Around my house, when I was growing up, ‘full of beans’ meant full of energy. But if my mother accused me of being ‘full of prunes,’ or ‘full of baloney,’ she meant I was acting foolish, or talking nonsense.
So I’ll stop now.