Where did the "unavailable" meaning of "Out of Pocket" come from? [duplicate]

The Oxford English Dictionary says out of pocket meaning "out of reach, absent, unavailable" dates back to the US of the early 20th century:

1908 ‘O. Henry’ Buried Treasure in Ainslee's July 69/2 Just now she is out of pocket. And I shall find her as soon as I can.

It's a novel use of a metaphoric idiom (with, needless to say, odd syntax).

Happens all the time, though this is the first I've heard this one. Where is this office located?

X is out of pocket normally means that expense has been occurred by X. Who is out of pocket? asks who incurred any expense; Geoff is out of pocket for this claims that Geoff was responsible for paying for this; If you wind up out of pocket, see me about payment is an invitation to submit a voucher for reimbursement. Needless to say, the money is currently unavailable to X.

The metaphor is that money comes out of one's pocket. The odd syntax is to say that the person is out of pocket instead of the money. But it's a fixed phrase and they do get weird.

In the usage cited, X is out of pocket has apparently been reinterpreted, with pocket being metaphorized as X's office rather than X's money -- money (and debt and responsibility) have disappeared entirely from the meaning. And it's X that's unavailable, not X's money.

Very interesting. Thank you.

I believe that the phrase out of pocket meaning “unavailable” has its origin in cell phone use. Being out of pocket meant being in an area where no cell tower was available, and therefore when out of pocket one was unreachable. For many years, there were many regional pockets not covered by the cellular telephone network. Martha’s Vineyard for years was notorious for lack of cell phone service. Thus, business executives traveling to Martha’s Vineyard would find themselves out of pocket. This cell-phone-specific meaning later morphed into using the phrase out of pocket to mean generally “unavailable, unreachable”.