expression, ichidan verb
repay meanness with kindness
Idiomatic expression meaning to respond to someone's ill will or harm with kindness or benevolence, often as a moral or magnanimous act.
彼は自分を裏切った相手に、仇を恩で報いるような態度を見せた。
He showed an attitude of repaying meanness with kindness toward the person who betrayed him.
The expression is composed of 仇 (あだ, harm/grudge), 恩 (おん, kindness/gratitude), and 報いる (むくいる, to repay). The exact historical origin is uncertain, but it reflects a traditional moral concept of returning good for evil.