出处：按学科分类—语言、文字 中国对外翻译出版公司《汉英成语词典》第22页（223字）
比喻人有难以预先料到的灾祸，例他刚才还是好好的，谁知天有不测风云，出门便遭遇车祸。
Fancy，all was well with him minutes ago．He steps out and like a bolt out of the blue，he is struck down and killed by a passing car．