Origin

This character combines the sickness radical on the left, which signals that the character describes an ailment or condition of the mind, with the character 知 (to know) on the right, which serves as a sound hint for pronunciation. The idea is that foolishness or stupidity is treated as a kind of mental illness or defect—a lack of proper knowledge or understanding. Over time, this combined form came to represent the meaning of being foolish or stupid, while the right component guides you toward how the character should be pronounced.