Young hip (primarily black) people use this to mean "out of line
," or behaving in a way that is unacceptable
under the circumstances.
Old business people use this to mean unreachable
by the usual means.
If you are both a hip black person and a business person, colleagues' use of this term in the latter context can be both ambiguous and amusing.
1) You ate the last hot wing off my plate? Yo, you out of pocket, son.
2) Sorry I'm just responding now, I was out of pocket all day yesterday.