you are dense

昨日江戸川マラソン（10kmラン）に参加しました。目標の1時間切りは果たせず1時間51秒でした。あと1分速ければ達成できたのに、残念。来月は世田谷でのランがあるので、再挑戦します。
さて、もし "You are dense." と言われたらどう思いますか？　桐野 夏生の "Grotesque" にこんな描写がありました。
When my parents argued it was unbearable. But Yuriko would just sit there nonchalantly watching TV. When my father and I fought she would sneak out of the room, but when our parents fought she didn't seem to care. Was she really that dense? Or couldn't she bear to watch my father and me fight?

ここの "dense" の意味が分からないので辞書を見ます。
・Oxford English Dictionary: informal (of a person) stupid; Even a relatively dense dad like myself could sense that something was wrong.
・Collins Dictionary: If you say that someone is dense, you mean that you think they are stupid and that they take a long time to understand simple things. [informal] He's not a bad man, just a bit dense.
・Cambridge English Dictionary: stupid: There are some really dense people in our class.

"dense" が人に対して使われると "stupid" の意味になるのが不思議に思えたので、何か説明は無いかと他の辞書もいろいろ見るとVocaburaly.comに次の記述がありました。
And if someone calls you dense, they think nothing can get into your thick skull.
頭蓋骨が厚過ぎるせいですか！