fatuousness

His fatuousness was evident when he believed the obvious prank.

彼が明らかな悪戯を信じたとき、その愚かさが明らかだった。

The speech was filled with such fatuousness that no one took it seriously.

そのスピーチはあまりにも愚かさに満ちていたため、誰も本気にしなかった。