A bolt from the blue
bolt一词指的`是霹雳，而blue则指碧蓝的天空，英语词典一般将其解释为：some thing sudden and unexpected，也就是“突如其来、始料不及”的意思。
在日常口语会话中，人们也常用 out of来代替介词from。
eg：
We had been sure she was in chicago, so her sudden appearance was a boltout of the blue。
我们大家都认定她现在在芝加哥，因此她的突然出现完全出乎我们的预料。
Miss anne has just got her salary raised，so her decision to resign was abolt from the blue.
