Fool that I am, I had always thought that the buck in passing the buck or the buck stops here was a dollar. Of course, when you think about it, that hardly makes sense. How would passing somebody a dollar shift responsibility? I probably thought that it was a dollar only because the elaphine alternative, that you were passing a male deer, was even less congruous. But that's etymologically closer.
Bucks have horns. The horns can be chopped off (it's best to kill the buck first, or at least get it drunk). Why would you chop off a buck's horn? To make a beautiful knife handle, of course. And once you have your buckhorn knife you can use it to cut things, stab people or mark the dealer in a game of poker.
The question of whose turn it is to deal is terribly important as the dealer can fiddle the cards, choose the form of poker, and has the advantage of placing the last bet. So in the saloons of the wild west and untamed Occident he needed to be clearly marked. This was done by stabbing a knife into the table in front of him. When his deal was done he would pass the buck(horn) to the next dealer.
Here is Mark Twain describing a conversation between a priest and a "rough" in the 1860s:
"Are you the duck that runs the gospel-mill next door?"
"Am I the-pardon me, I believe I do not understand?"
With another sigh and a half-sob, Scotty rejoined:
"Why you see we are in a bit of trouble, and the boys thought maybe you would give us a lift, if we'd tackle you - that is, if I've got the rights of it and you are the head clerk of the doxology-works next door."
"I am the shepherd in charge of the flock whose fold is next door."
"The spiritual adviser of the little company of believers whose sanctuary adjoins these premises."
Scotty scratched his head, reflected a moment, and then said:
"You ruther hold over me, pard. I reckon I can't call that hand. Ante and pass the buck."
"How? I beg pardon. What did I understand you to say?"
"Well, you've ruther got the bulge on me. Or maybe we've both got the bulge somehow. You don't smoke me and I don't smoke you. You see, one of the boys has passed in his checks and we want to give him a good send-off, and so the thing I'm on now is to roust out somebody to jerk a little chin-music for us and waltz him through handsome."
"My friend, I seem to grow more and more bewildered. Your observations are wholly incomprehensible to me. Cannot you simplify them in some way? At first I thought perhaps I understood you, but I grope now. Would it not expedite matters if you restricted yourself to categorical statements of fact unencumbered with obstructing accumulations of metaphor and allegory?"
[In case you're wondering, Scotty's friend has died and he wants the priest to deliver the eulogy at the funeral]
Anyway, a chap called Fred Canfil visited the warden of a prison in Oklahoma and saw a sign above the desk saying The Buck Stops Here. He liked it so much that he had a copy made and gave it to his boss Harry S. Truman who put it up in his office.
Which leads us, dear reader, to one overwhelming question: what was Harry S. Truman's full name?
I'll give you a clue: it's the same principle as Ulysses S. Grant and H. Norman Schwarzkopf.
Harry S. Truman's full name was Harry S. Truman.
The Inky Fool discussing grammar
N.B. The OED is cautious about the buckhorn knife theory, but I think it holds good.