Something happens so suddenly that one has no time to defend or prepare.
猝突然，出其不意。事情来得突然，来不及防备。
清·纪昀《阅微草堂笔记·姑妄听之一》既不炳烛，又不扬声，猝不及防，突然相遇，是先生犯鬼，非鬼犯先生。”
The storm arrived so quickly that everyone was caught off guard.
Learn the next connected idiom: 猝不及防 (cù bù jí fáng) → 防不胜防 (fáng bù shèng fáng)
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