hard feelings; mutual resentment
Almost always used in negative constructions like 恨みっこなし or 恨みっこなしだよ, meaning 'no hard feelings' or 'let's not hold grudges'. A casual, somewhat childish word.
No hard feelings even if you lose.
お互いさま、恨みっこなしにしよう。
It's mutual, so let's not hold any grudges.