恨む
うらむ · uramu · JLPT N2 · verb-godan
to feel bitter
ura = hidden inner heart; uramu = to nurse a grudge in the heart
Old Japanese ura (心/裏) meant the hidden inside, the heart you keep to yourself. Add the verb ending -mu and uramu means to hold a bitter feeling inside. The kanji 恨 agrees: 忄 heart plus 艮, which means stubborn, stuck in place.
恨 — heart radical 忄 + 艮 (gěn, phonetic)
恨 is a semantic-phonetic compound. The heart radical 忄 (the side form of 心) signals an emotion or feeling, while 艮 supplies the sound (related to 'gěn/hèn'). Together they convey a strong negative feeling held in the heart — resentment, hatred, or deep regret.