Heavens to Mergatroyd! I go out for a little while to smoke a cigar and work on my speech and come back to find an encyclopedia on the phrase “out of pocket.” A few examples:

OUT OF POCKET – “Used in the Southwest for ‘absent, unavailable.’ ‘I’ll be out of pocket awhile, but I’ll call you as soon as I can.” From the “Happy Trails: Western Words and Sayings” chapter in the “Facts on File Dictionary of American Regionalisms: Local Expressions from Coast to Coast” by Robert Hendrickson (Facts on File, New York, 2000).

And:

On “out of pocket” –
Not sure about the military, but a QB who goes “out of the pocket” is no longer a QB in the strictest sense, and can rush beyond the line of scrimmage and still throw like any other player (which is illegal for a QB who is in the pocket). Going the other way, defenses try to drive the QB out of the pocket because then he will likely make more mistakes. (Unless his name is Favre.)
So, I would fairly interpret “out of [the] pocket” to mean, driven out of the normal place by unusual circumstances, and so normal rules (like instant communications) don’t apply.

And:

The first time I remember hearing the phrase was when Kristin Scott
Thomas uttered it in the first Mission Impossible movie.
So, ever since, I associate it with spies and foggy Prague streets.

And:

Jonah:
The term is used all the time on Wall Street and in Big Law. Means not (immediately) reachable/available.

And finally, a reader sent me an excerpt from this discussion:

From Daniel:
I wonder if you have any insights into how and when the phrase out of pocket has come to mean “unavailable” or “away from the office”. It’s so common now that when I use the phrase in (what I believe to be) its original sense of “requiring one to pay cash that may not be reimbursed”, I sometimes cause confusion. Is this a generational issue, or a regional one, or…? Whence this troublesome usage? Out of pocket originally referred to being out of funds. The OED’s earliest example of this is from 1693. That meaning is considered obsolete. It also meant “to be a loser in a transaction” in the 18th century. By 1885 it was being used to refer to expenses that were not covered or that would not be reimbursed by another source. This was the dominant meaning until the 20th century, when, as you mention, the meaning “away” or “unavailable” arose. Interestingly, the OED’s earliest citation for this usage, which they say is American, is from 1974. Once again, the amazing TOWFI research team has found a citation almost 30 years earlier, from 1946 (another WOOHOO!). We reproduce it for you in its entirety below:
Law Officers Request Aid in Uncovering Texarkana Murderer Texarkana, May 11 —Murder-haunted residents in the Texarkana area were asked to play detective today in an effort to track down the slayer of five persons during the past seven weeks. Sheriff W. H. Presley and Chief of Police Jack Runnels asked “every man and woman in these two counties to recall whether or not any person close to them was missing or out of pocket” during the nights when the killer stalked his victims. Asserting somebody was “out of pocket” the nights of the slayings in Bowie and Miller counties, the officers appealed to the residents for information. – Salamanca [New York] Republican-Press, May 11, 1946 Our best guess is that the existing out of pocket was adopted to mean “away”, “missing”, or “not in contact”, probably with the influence of the phrase in [someone’s] pocket, which had been around since the early 19th century, with the meaning “quite close to, in close attendance upon [someone]“. We also suspect that, if the phrase with the “missing” meaning appeared in a newspaper of 1947, it had been in use for some time prior to that.