Birds Startled by the Mere Twang of a Bowstring

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.'

This idiom means that if one has been frightened in the past one's may become paralyzed in a similar situation