The shallowness of his speech showed a lack of deep thinking.

shallowness 是由形容词 shallow（浅的、肤浅的）加上名词后缀 -ness 构成的名词，表示"浅薄、肤浅"的状态或性质。shallow 本身由古英语 sceald（浅的）演变而来，-ness 是常见的将形容词转化为抽象名词的后缀。