expression
with no hard feelings; no grudges
Used when agreeing to let go of resentment, often after a competition or dispute. Common in phrases like 恨みっこなしだよ or 恨みっこなしにしよう.
勝負は勝負、恨みっこなしだ。
A match is a match; no hard feelings.
お互いさま、恨みっこなしにしよう。
We're even, so let's not hold any grudges.
恨む is the verb 'to bear a grudge'; 恨みっこなし is a set phrase meaning 'without holding grudges'.
From 恨み (grudge) + っこ (suffix indicating mutual action) + なし (without). The っこ element adds a nuance of reciprocity.