出其不意 take someone by surprise
Explanation
出其不意是一个汉语成语，意思是趁对方没有意料到就采取行动，也指事情发生得突然，让人没有防备。它源于《孙子兵法·计篇》中“攻其无备，出其不意”。
To take someone by surprise, it is an idiom which means to take action when the other party does not expect it. It also refers to events happening suddenly and catching someone off guard. It is originated from the book "The Art of War" by Sun Tzu: "Attack when the enemy is unprepared and take them by surprise.
Origin Story
战国时期，齐国与燕国发生战争，燕国将军秦开率领军队攻打齐国。齐国将军田单早已料到燕国会来犯，就暗中制定了一个计划。当燕国军队到达齐国边境时，田单命令士兵假装逃窜，引诱燕军深入齐国境内。燕军看到齐军溃败，以为大胜在望，便急急忙忙地追击，一路上没有设防，放松了警惕。田单趁机率领精锐部队从侧面发起进攻，燕军措手不及，大败而逃。田单用这一招“出其不意”，取得了战争的胜利。
During the Warring States period in China, a war broke out between the State of Qi and the State of Yan. Qin Kai, the general of Yan, led his army to attack Qi. Tian Dan, the general of Qi, had already anticipated that Yan would attack, so he secretly formulated a plan. When the Yan army reached the border of Qi, Tian Dan ordered his soldiers to pretend to flee, luring the Yan army deep into Qi territory. The Yan army, seeing the Qi army retreating, thought a great victory was in sight, and hastily pursued them, leaving no defenses along the way and letting down their guard. Tian Dan took the opportunity to lead his elite troops in a flanking attack. The Yan army was caught off guard and fled in defeat. Tian Dan achieved victory in the war by using this tactic of “taking the enemy by surprise.”
Usage
这个成语主要用于军事领域，形容突然袭击，攻其不备，但也常用于日常生活中，形容做事情突然，出乎意料，让人措手不及。
This idiom is mainly used in the military field, to describe a sudden attack, to attack when the enemy is unprepared. But it is also often used in daily life, to describe something that happens suddenly and unexpectedly, catching someone off guard.
Examples
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将军指挥部队出其不意地包围了敌军。
jiang jun zhi hui bu dui chu qi bu yi di bao wei le di jun.
The general led his troops to a surprise attack on the enemy army.
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他做事总爱出其不意，让人措手不及。
ta zuo shi zong ai chu qi bu yi, rang ren cuo shou bu ji
He always does things unexpectedly, catching people off guard.