The story
From outer (“exterior, seen from China proper”) + Mongolia.
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: Synonym of Mongolia, (country); viewed as a region south of Siberia, west of Manchuria and north of Inner Mongolia (with closely related native Mongolian population), well north of the Great Wall, which never became an integral part of the Chinese empire, and later achieved autonomy and full independence from China, as the republic of Mongolia. Full definition →
From outer (“exterior, seen from China proper”) + Mongolia.
Etymology from Wiktionary, CC BY-SA 4.0.