A blocked passage in one's heart (see Notes)
To not understand a single thing that's going on; to be exceptionally ignorant. Often used to ridicule others' foolishness or stupidity.
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A couple dictionaries are indicating that the ancient Chinese believed that people were capable of thought because of the existence of a passageway in the heart. If the passageway were blocked, as indicated in the chengyu, then people would be unable to think properly
《吕氏春秋》(Lǚ Shì Chūn Qiū), a compilation of the philosophies of the Hundred Schools of Thought. It was created during the Qín Dynasty (秦, 221 - 207 BC) with the patronage of Chancellor 吕不韦 (Lǚ Bù Wéi)
纣王 (Zhòu Wáng) was the emperor of the ancient Shāng Dynasty (商, 1600 - 1046 BC). He was an exceptionally violent ruler, and he often ignored affairs of state to spend time with his favorite concubine, 妲己 (Dá Jǐ).
There was a loyal sage in the royal court named 比干 (Bǐ Gān) who despaired at the emperor's behavior. He repeatedly entreated the emperor to change his ways, but he was repeatedly ignored.
Once, the emperor killed another loyal official at the behest of his concubine. When 比干 heard the news, he grew greatly alarmed, and he again went to plead for the king to change his ways and prevent the country from suffering.
比干's repeated actions deeply angered the emperor, and he snarled, "I've always heard that sages have seven passageways in their hearts. I'm going to kill you and see whether this is true."
And, true to his word, he had the official executed and his heart torn from his chest.
Later, 孔子 (Kǒng Zǐ, or Confucius) would use this story in his teachings, lamenting at the Zhōu emperor's inability to think clearly.