noun
within one; at heart
Literary or formal expression for what is inside a person's mind or heart; often used in phrases like 腹中を探る (to probe someone's thoughts) or 腹中を見抜く (to see through someone).
彼の腹中は誰にもわからない。
No one knows what is in his heart.
腹中を探るような質問はやめてください。
Please stop asking questions that probe my inner thoughts.