at the end of the qin dynasty(221-206 b.c.) the state of chu and the state of han fought for control for the country. xiang yu, the king of chu, was besieged1（包围） at a place called gaixia by the han army led by liu bang. xiang yu was in a desperate situation, with little food and only a few soliders. at night, the surrounding han troops started to sing chu folk songs. xiang yu was very surprised at this, and said:"has liu bang occupied the whole of chu? how can he have drafted so many chu people into his army?" then he fled together with the remainder of his forces.
this idiom is used metaphorically2 to mean to be in a *less and critical situation, surrounded by the enemy on all sides.
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