There are many mountains called East Mountain, one in Guiyang, one in Shandong, and there's one outside Jiankang as well.
During the Eastern Jin period, a young man named Xie An who was unsuccessful in his political career, retired to the East Mountain in Huiji and stayed there until he was forty-one years old before he returned to Jiankang (then known as Jianye) to take an official position. He resided on a mountain outside the city and named it East Mountain.
About eighty years earlier, under his command, the Eastern Jin had won a decisive battle with fewer troops against many, which not only temporarily stabilized the north-south situation but also secured his status at the court. From then on, he was influential in both the government and the court and eventually became the Prime Minister.
The idiom about East Mountain was thus coined based on this historical reference.
