This idiom originates from the
史记. When asked why King Wei of Chu had ignored state affairs for three years, a courtier replied: 'That bird has not flown, but when it flies it will soar to the sky; it has not sung, but when it sings it will astound people.' (
此鸟不鸣则已,一鸣惊人). King Wei then reformed and became a great ruler.