In the Warring States Period, there was a man in the State of Wei called Geng Lei. One day he said to the king: 'I can shoot down birds by simply plucking my bowstring.' When the king expressed doubt, Geng Lei pointed his bow at a wild goose flying in the sky, twanged the bowstring, and the goose fell to the ground. Geng Lei said, 'This goose has been hurt in the past. Hearing the twang of the bowstring, it assumed that it was doomed. So it simply gave up trying to live.
战国时期（公元前403―221年中国中原地区各诸侯国连年争战的时代）魏国有个名叫更羸的人。一天，他对国王说：“我只要拉开弓，空射一下，就能把天上的鸟射下来。”国王不相信。更羸便对准天上飞来的一只雁射去，果真那只雁听到拉弦的声音就掉了下来。国王感到很奇怪。更羸说，“那是一只受过伤的雁。它一听到我拉开弓弦的声响，就惊慌得支持不住，自然要掉下来了。”

This idiom means that if one has been frightened in the past one's will may become paralysed in a similar situation.
“惊弓之鸟”这个成语比喻受过惊恐之后，有一点动静就特别害怕。