九世之仇

Etymology
From the story of two ancient states during the Western Zhou dynasty:

Duke Ai of Qi had a dispute with his neighbouring state Ji (紀). King Yi of Zhou sided with Marquis of Ji and executed Duke Ai. Later, Duke Xiang of Qi, the ninth-generation descendant of Duke Ai, conquered the State of Ji to revenge for him.



Idiom

 * 1) irreconcilable resentments from generations of hatred since the remote past