His foolishness of wearing pajamas to the meeting was laughable.

foolishness 是由形容词 foolish（愚蠢的）加上名词后缀 -ness 构成的名词，表示"愚蠢的行为或状态"。后缀 -ness 通常用于将形容词转化为名词，表示某种性质或状态。