Tapping out

What's really been bothering me lately is the expression "tapped out." Where does it come from? I always assumed that it came from wrestling: when a man is pinned and gives up unpinning himself, he taps the other wrestler to indicate his surrender. But then, when you come to the end of a long grey day and realize that you haven't the strength for even one more useful task, shouldn't you say, "I've tapped out," or "I'm tapping out," as opposed to what one does say, which is "I'm tapped out."

That grammar would indicate that the etymology (can you have etymology of a phrase?) is rooted in a beer keg. When you pour beer from a keg you tap it, and when it's empty it's tapped out, right? (obviously, I'm way out of my depth here) Then the conjugation makes sense, because when you say "I'm tapped out" you are just substituting yourself for the beer keg, you being empty of energy, not beer.

But three dictionaries neither confirm nor deny this hypothesis (including Canadian Oxford!) and the definition of an expression is distressingly hard to Google. Now I'm worried I've made the whole thing up, and in fact no one says "tap out" in any context, ever, except me!

You know, I think I am. Tapped out, I mean. It's been a really long week. Perhaps I'll leave this question for better minds than mine, and go do something fun.

Can you bring me back a cardinal from Kentucky?
RR

PS--My orthodonist unexpectedly announced this morning that I don't have to wear my retainer during the day anymore. Unexpectedly because he'd said before that it would be maybe as much as six months before I'd have this luxury. I think he gave me the worst case scenario because he knows I don't take disappointment too well, but I take unexpected good news very well. Despite my exhaustion, I am ebullient. I went right to the dep and bought gum, Raspberry Extra, which is repulsive, but which I am chewing right now for the lack of anything else. On my way to the fun, I'll buy something better.