"Out of pocket"?

I'm increasingly hearing the phrase "out of pocket" used in America as a colloquialism to mean "away from the office", "unavailable", or "incommunicado".

I apologize for not replying sooner; I have been out of pocket.

• What is the etymology of this usage?
  It always sounds odd to me, since I had previously always associated the phrase with expenses, e.g., "I had to pay for the service out of pocket," meaning, "I had to pay for the service using my own money."
• Should either of these usages be avoided?
• What is the extent of their acceptance?

Here's a cite from the OED:

1946 Sunday Times-Signal (Zanesville, Ohio) 12 May I. 7/1 [They] told citizens here that somebody was ‘out of pocket’ in Bowie and Miller counties the nights of the killings, and urged them to recall whether anyone they knew was missing on those dates. 1973 J. PETERSON Sicilian Slaughter 53 Her hands shook as she dialed. But her connection was out of pocket. 1974 Anderson (S. Carolina) Independent 20 Apr. 1A/1 If you..have ever been sick and the only doctor is out of pocket for the weekend, then you know we need more doctors. 2002 A. PHILLIPS Prague III. viii. 229 Five-day weekend for me, Charlie, starting in eighteen minutes. I'll be out of pocket until Tuesday.

I first heard this in the US Southern states, and some attest to it being common on the East Coast. This is a regionalism, and while quite old, it should be considered informal and would not be universally understood by all US speakers. Those on the West Coast, for example, would be largely unfamiliar with it, and only use "out of pocket" to refer to self-expense payments.

The OED has been since updated and their earliest citation is in a 1908 story by ‘O. Henry’ (real name William Porter) called Buried Treasure, published in Ainslee's:

Just now she is out of pocket. And I shall find her as soon as I can.

It means you're out of reach or unavailable, and one suggestion is it's the opposite of in [someone's] pocket, which means you're under someone's control or influence. Alternatively, it may be the opposite of another phrase, to put (or keep) in [someone's] pocket, meaning to keep something for yourself or conceal. But this is speculation and the real origin is unknown.

In BE at least I haven't heard of it other than talking about money and specifically money that you should have got.

Out of Pocket means you had to pay for something yourself, or you suffered financially on some arrangement.

edit:
I can't find "out of pocket" or "out of place" mentioned in any email system in place of "out of office". There is a common "Oops ..." error message when a mail couldn't be delivered but I can't see how that is confused.

I am from the South, but I didn't think that it was a regional expression, until I just used to explain that I wasn't able to meet someone, and my correspondent - who is in British Columbia - thought that I had financial difficulties.

The expression always made sense to me, from the first time that I heard it, because in the rural areas, it is sometimes difficult to get from Point A to Point B. When you are in an isolated area, far from a major - or even medium sized city, it makes it difficult to get to other places. These isolated areas could be considered "pockets" because they are hard to get out of and so "out of pocket" refers to the difficulty to get from one location to another.

It is a helluva lot easier for say, someone from New York to hop on a plane to get to Paris than it would be for say someone from Long Key, in Florida to get to Orlando.

To me, "out of pocket" would not refer to being unavailable in an office, but more or less being in a different geographical area (pocket) that would be too inconvenient at the time to try to make it.

It's like if someone were in Mato Grosso do Sul in Brazil and there is a meeting in Adelaide, Australia, then that person would have to fly from Mato Grosso do Sul to Sao Paulo and from there to Miami and then to Atlanta and connect to Los Angeles and from there to Sydney and then to Adelaide by which time 36 hours have passed and the meeting would already be over - that would DEFINITELY be considered "out of pocket".