She nodded with a knowingness that clearly hinted she was aware of more than she let on.

knowingness 是由动词 know（知道）加上后缀 -ing 变成形容词 knowing（心知肚明的），再加上名词后缀 -ness 构成的名词，表示"心知肚明的状态"或"了然于心的神情"。