刻舟求剑（kè zhōu qiú jiàn）

刻舟求剑（kè zhōu qiú jiàn ；mark boat seek sword）

Once upon a time, a gentleman with a sword rode a ferry across a river. Unfortunately, when they reached the middle of the river his sword unexpectedly fell into the water. This gentleman immediately made a mark on the edge of the moving boat and said “This is where my sword fell in.” after the ferry reached the far back of the river, he jumped into the river to look for his sword under the spot where he made the mark earlier. Of course, he was unable to find the sword.

This idiom is used to describe a person who is ridiculously rigid and takes measures without regard to changes in circumstance.