A couple of posts back, I mentioned a published list of Britishisms that included eleven “words and expressions that have been common in America for as long as I can remember, and which I wouldn’t have necessarily thought of as British in origin.” I commented that initial investigation suggested only three of them seemed to be proper Britishisms. The first was “smarmy.” And the second is “full of beans.”
The OED defines the expression, which seems to derive from horse racing, as meaning “to be full of energy, and in high spirits.” The first citation is from an 1843 novel: “‘Ounds, ‘osses, and men, are in a glorious state of excitement! Full o’ beans and benevolence!”
That and all subsequent citations are from British sources. In Green’s Dictionary of Slang, all cites are from Britain or the Commonwealth until this from the American writer Leo Rosten’s 1975 novel Dear Herm: “Now he is full of beans and vinegar and with a whole new outlook on Life.” (That seems like a euphemistic switch on the roughly equivalent U.S. phrase “full of piss and vinegar.”) One earlier U.S. use of “full of beans” is in a 1938 New York Times article: “Whenever Sage, a cowboy with whom I once punched cows on the San Simon Ranch in Eastern New Mexico, felt particularly full of beans of a cool early morning….”
In any case, Google Books Ngram Viewer shows British dominance for the phrase until roughly the late 1970s, when the U.S. caught up. That was followed by a British spurt, and equivalence again in 2000, the last year for which the database has reliable data. (Note there are some false positives, for example, for references to a pot that is literally full of beans.)
notoneoffbritishisms.com 
May we all be full of beans and benevolence!
My mother used this phrase often when I was growing up in the late ’60’s/early 70’s. It meant “full of energy”, with a side of silly or frivolousness . She was raised in the western PA/eastern OH area, for those keeping score at home. First time I really heard it elsewhere was in the movie “Unforgiven”.
Would the 1843 novel be by Surtees?
Yes: “Handley Cross; or, The Spy Hunt”
I have also often heard “full of beans” used in the USA as a euphemism for “full of s***t.”
That’s the only way I’ve ever heard it in the U.S.
Same here. I wonder if this was an evolution after people heard the expression used in a positive way and, not having a clear context, assumed it was a euphemism for the negative phrase. I have also heard “full of prunes” (doubtless due to the association between prune juice and defecation – and if we take “full of beans” as referring to baked beans rather than green beans, there is an obvious connection there as well).
I have no idea whether this is relevant, but this ditty from Little House on the Prairie popped into my head:
I’m Captain Jinks of the Horse Marines
I feed my horse on corn and beans,
And often live beyond my means
Tho’ a Captain in the Army.
Received this just today: Headline: “NRA Is Full of Beans, Claims Sculpture Artist in Copyright Suit” Anish Kapoor, sculptor of the Chicago landmark “Cloud Gate,” aka “The Bean” sculpture is demanding the NRA stop using pictures of it in its advertising. Context suggests reporter sees “full of beans” as lying (or is just looking for cheap wordplay): https://www.adlawbyrequest.com/2018/11/articles/in-the-courts/nra-is-full-of-beans-claims-sculpture-artist-in-copyright-suit?utm_source=Reed+Smith+-+AdLaw+By+Request&utm_campaign=9b5c03157c-RSS_EMAIL_CAMPAIGN&utm_medium=email&utm_term=0_e842abcb2d-9b5c03157c-70801217
It was always explained to me that a horse that had been fed beans, rather than just corn or grass, would be more energetic.
“Britain or the Commonwealth” – that’s a bit like saying “France or Europe” or “Oregon or the west coast”. Britain is part of the Commonwealth.
I grew up in Southeastern PA in the ’80s. Full of beans meant lively or overly excited.