"out of pocket" (= not reachable)

[Gerald Cohen]

    Thanx for the responses on "loose as a goose." BTW, I'm curious
about the source of Patton's quote.

    Also, today I received a query on the expression "out of pocket"
as used to mean "really not reachable, really not available"; e.g.
"The director will be out of pocket for the next two hours." I.e.,
even by cell phone he can't be reached. FWIW, I had never heard it
used in this sense before. The person who sent me this query works in
the financial community in NYC.

    Would anyone know the original reference of this "out of pocket?"
Is it figuratively a pants pocket? The football pocket to protect the
quarterback?
Something else?

    Any information/insight/leads would be very much appreciated.

--Gerald Cohen