na-adjective, noun
knowing look; look of understanding
Describes a facial expression that shows one is in the know or has grasped the situation. Often used in literary or descriptive contexts.
彼は心得顔でうなずいた。
He nodded with a knowing look.
その話を聞いて、彼女は心得顔の表情を浮かべた
Hearing that, she put on a look of understanding.