From 《史记·范睢蔡泽列传》: 一饭之德必偿，睚眦之怨必报 - 'repay every meal's kindness, avenge even the slightest glare'. Originally describing Fan Ju of the Warring States; now almost always pejorative.
memory
睚眦 literally = the angry glare you get from a stranger. 必报 = will definitely take revenge. Even a dirty look will be repaid.
