外强中干

Origin and story

Zuozhuan, in Duke Xi's fifteenth year, records Qing Zheng warning the ruler of Jin about the horses chosen for battle against Qin. They had come from Zheng, and Qing Zheng argues that unfamiliar horses may panic under battlefield pressure. He describes a misleading display of strength: muscles and veins tense, while inner energy fails and the animals cannot advance, retreat, or turn as required. The ruler ignores him and is eventually captured. This is the historical text's account and ancient explanation of the danger, not a modern veterinary rule about foreign horses. The idiom broadens the contrast to anything impressive outside but weak within.