腹 as seat of emotion

In traditional Japanese thought, the 腹 (hara, belly) — not the heart — was considered the center of true feeling, intention, and character. This survives in many fixed expressions: 腹を立てる (get angry), 腹を割る (speak frankly), 腹黒い (malicious), 腹を据える (steel oneself), and the historical notion of 切腹 (seppuku) as a way to display the 'true self' within.