na-adjective, noun
Literary or old-fashioned term for mental dullness or slowness; not common in everyday speech.
彼は痴鈍な顔つきで、何を考えているのかわからなかった。
He had a dull-witted expression, and I couldn't tell what he was thinking.
その老人は痴鈍を装って、余計な質問を避けた。
The old man feigned dullness to avoid unnecessary questions.