Outer Mongolia was a region located in Asia. The area was located within the borders of the nation-state of Mongolia and had once been part of the Chinese Empire. Outer Mongolia was also used as a slang term denoting something that is impossible to achieve.
After traveling back to 1930 to await the arrival of Leonard McCoy, James Kirk referred to Outer Mongolia as one of the countless locations McCoy might arrive in – as well as clarifying the impossibility of finding him in a seemingly random situation. (TOS: "The City on the Edge of Forever")