A one-sided grudge — resentment held by one person toward another that is not reciprocated or resolved.
過去の出来事が原因で彼は片恨みを抱いている。
He holds a grudge because of something that happened in the past.
片恨みは人間関係を長くこじらせることがある。
A one-sided grudge can sour relationships for a long time.
小さな迷惑をいつまでも片恨みにするのはよくない。
It's not good to hold a grudge forever over a small annoyance.